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reference the out-of-date fork.

Fixes #55

- Three configuration directives have been added: LDAP_OPENLDAP,
 LDAP_GROUP_MEMBER_FILTER, LDAP_GROUP_MEMBER_FILTER_FIELD
- Examples: basic_auth/app_oldap.py and groups/app_oldap.py
- Updated README.md with a more instructions

closes #16
closes #15
closes #14

to link to the new example using blueprints and app factories