Issue #73: group_required not working as expected

- 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reference the out-of-date fork.

Referral chasing crash

Bind ldap to current app

get_object_details to take query_filter and fallback to LDAP_USER_OBJECT_FILTER or LDAP_GROUP_OBJECT_FILTER

 next vaule: Priority use request.full_path

OpenLDAP section has Incorrect LDAP_GROUP_OBJECT_FILTER

Fixes #44 - Error in bind_user method

 to LDAP_USER_OBJECT_FILTER or LDAP_GROUP_OBJECT_FILTER
fixes #58

login redirect next parameter Priority use request.full_path。 full_path contains url params

Fixes #55