PostgreSQL tests

I started to add tests for PostgreSQL and already fixed some issues. Though, some tests still fail. From my point of view, it is not safe to use PostgreSQL at the moment.  
This [test](https://github.com/fruux/sabre-dav/blob/380d6cea83e657a5fd5a89bed126418ec8742f78/tests/Sabre/DAVACL/PrincipalBackend/AbstractPDOTest.php#L121) indicates that the search should be case insensitive. PostgreSQL by default will always do case sensitive comparisons. You can specify the according field as [citext](http://www.postgresql.org/docs/9.3/static/citext.html) in the sql schema, but since citext is an optional module, it's not always available. Therefore we should not rely on citext.

Sabredavs MySQL schema uses ``utf8_unicode_ci`` which is case insensitive by default. (I think ``utf8_bin`` would be case sensitive.)

To overcome the different sql drivers case sensitivities, I used ``mb_strtolower`` to make strings always lowercase.