support ldap3

[thraxil]

python-ldap doesn't work with python3 and is apparently stalled out.

This adds transparent support for ldap3, which is python3 compatible and will let us move forward on that.

As implemented here, if the ldap3 library is importable, it uses that. Otherwise, it falls back to python-ldap.

My suggested plan is to get this out in a new release. Then we can go app by app adding ldap3. Once all of our applications are using that, we can remove the old python-ldap support.

some additional notes:

• some of the varialble names are weird camelCase. that's a bit of a holdover from the old ldap code, which I decided to stick to as closely as possible at first. I think when we get rid of the python-ldap stuff, that can be cleaned up and modernized.
• the ldap stuff should probably be completely split out of djangowind into its own package. Again, that can happen after the transition.
• I didn't quite have it in me to mock out ldap to properly test that functionality. So I've just been manually testing that bit for now. Eventually though...

[coveralls]

Coverage decreased (-1.2%) to 75.2% when pulling c91ee9b on ldap3 into b7b6d3f on master.

[sdreher]

Will this still result in an ImportError exception? self.ldap_lookup will just be undefined here.

[thraxil]

Yep. Forgot the default implementation...

[coveralls]

Coverage decreased (-1.3%) to 75.057% when pulling b13d75c on ldap3 into b7b6d3f on master.

[sdreher]

Cool. And, can this try/catch be removed as well?

[thraxil]

yep

[coveralls]

Coverage decreased (-1.07%) to 75.314% when pulling 465a93e on ldap3 into b7b6d3f on master.