BUGFIX: Extensible Match Filter based on DN attributes not working.

I noticed there was a bug in the extensibleMatch filtering (ref https://tools.ietf.org/html/rfc4511#section-4.5.1).  I added the code to correct and a couple of unit tests.

Thanks,
Carl
Catching everything is a bad practice. Please at least specify the base class ``Exception``.
The Python docs have some more info on why this is bad: https://docs.python.org/2.7/howto/doanddont.html#except