Handle empty search results w/references

corrected init config for certificate path setting

Adds get_groups method to provide an interface for

updated to get proper groups

Issue #73: group_required not working as expected

If a search result is empty but there are result reference locations
that are returned, then the current code will return invalid information
or throw an AttributeError.

Address issue #70.

The search result would be in the format a dn string and then a dictionary
containing the proper information about the user, but when the search is
empty you just end up with the search references as shown below, and
obviously the second item in each tuple is not a dict:

[(None, ['ldap://ForestDnsZones.mycompany.com/DC=ForestDnsZones,DC=mycompany,DC=com']),
 (None, ['ldap://DomainDnsZones.mycompany.com/DC=DomainDnsZones,DC=mycompany,DC=com']),
 (None, ['ldap://mycompany.com/CN=Configuration,DC=mycompany,DC=com'])]

A successful search looks something like this:
[('CN=My Name,OU=Users,OU=Accounts,DC=mycompany,DC=com',
 {'accountExpires': [b'9223372036854775807'],
  'cn': [b'First Last'],
  'userPrincipalName': [b'email@mycompany.com'],
  'whenChanged': [b'20210119003635.0Z'],
  'whenCreated': [b'20201005164610.0Z']}),
  (None, ['ldap://ForestDnsZones.mycompany.com/DC=ForestDnsZones,DC=mycompany,DC=com']),
  (None, ['ldap://DomainDnsZones.mycompany.com/DC=DomainDnsZones,DC=mycompany,DC=com']),
  (None, ['ldap://mycompany.com/CN=Configuration,DC=mycompany,DC=com'])

]

Signed-off-by: Fred Thomsen <me@fredthomsen.net>

Signed-off-by: Nemanja Milakovic <nemanja@thingsolver.com>

listing all available groups in the ldap directory
based on the filter defined in LDAP_GROUPS_OBJECT_FILTER

The get_groups method has two arguments fields and dn_only
to customize the result. i.e.
get_groups(fields=['cn', 'description'])
provide a list of dict with included attributes. if only
get_groups(dn_only=True) is provided, a list of available
groups dn is returned. If no users in directory, an empty
list is returned.

- fixes #73

Don't include /? in the next argument when user comes in to the index

Fix regression in groups-related functions (".." vs b"..")

Improve the README for the first-time users

(get_user_groups/get_group_members/group_required)

This was undone accidentally when removing support for Python2 in
https://github.com/admiralobvious/flask-simpleldap/commit/b2c0d10332d27db8b349753f89dceb7db520b7f2#diff-df961969fdc0fcec2f458e0fbb16eef9L238

Closes #63, #64.

Note the default behavior of `LDAP_USER_OBJECT_FILTER`.

Compare the groups and the blueprints examples, and note that the
groups examples additionally shows how to use `@login_required` (this
was hard to find!)

As noted in:
https://github.com/admiralobvious/flask-simpleldap/issues/5
https://github.com/admiralobvious/flask-simpleldap/issues/43

- @login_required can't be used without like that, so it makes sense to
  replace it with the more basic `@basic_auth_required`.
- It's easy to forget to configure `LDAP_HOST`, and many people probably
  are trying to connect to a pre-existing directory, instead of bringing
  up their own.

As demonstrated by the 230 upvotes and 180k+ views on
https://stackoverflow.com/questions/4768446/i-cant-install-python-ldap
- it doesn't usually install the first time.

As the README didn't mention the prerequisites, I immediately turned to
Google after encountering an error, which was less helpful than the
official docs.

(I moved the Python compatibility info to another section, as it looked
out of place after I added the link to python-ldap installation
instructions.)

Follow-up to 5455333d and
bd69525c.

The pyldap link pointed to an archived repository for a fork that has
been merged into mainline python-ldap package as of 3.0. This package
specifies python-ldap>=3.0.0 as dependency, so there's no need to
reference the out-of-date fork.