Revert "Add exit() function to unbind"

This reverts commit 41f38a6e044811ee5dda5a05b3c60546c7c3f564.
Revert "Merge pull request #4 from mgorny/bind_as-fixes"

This reverts commit 0cfd223d85b161582015505156e9bae77b73268d, reversing
changes made to 88df2cc1b6288ac35d5e0c529548ef8a4618495f.
Revert "Merge pull request #3 from mgorny/bind_as"

This reverts commit 88df2cc1b6288ac35d5e0c529548ef8a4618495f, reversing
changes made to 98fb017dac1fe68abc7eddcaa6f4a2383329b6a1.
Support calling build_rdn() and build_dn() on a class.

This makes it possible to obtain the DN for an LDAP object without
having to fetch from LDAP first.

Conflicts:
	ldapdb/models/base.py
Wrap up connection obtaining code in _get_connection().
Support binding to a different database alias using bind_as().

Conflicts:
	ldapdb/models/base.py
Support restoring settings.DATABASES after bind_as().
Add a context manager interface for bind_as() + restore_alias().
Close LDAP connections properly.

Implement the close() method in DatabaseWrapper so that the connection
to LDAP is actually unbound and closed properly as necessary.
Merge pull request #6 from mgorny/okupy

Update to fit a bit more into current ldapdb status