Upgrade to Python 3 (and keep compatibility with Python 2)

[GuillaumeDerval]

By switching from python-ldap to pyldap (which is a fork compatible with Python 2/3 that is well maintained, and offers nearly the exact same API as python-ldap), I was able to update the source code to something compatible with both Python 2.6+ and 3.3+.

All tests pass, at least on Python 2.7 and 3.5.