[#55] Allow CSW harvesters to define CQL filters (2nd try)

Replaces #56: this branch should merge properly on master.

As per #55 (Allow CSW harvesters to define CQL filters):
* changes in py files to allow passing cql info
* changes in owslib version, to make the cql filtering work (some improvements were introduced in 0.8.4 and some more fixes in 0.8.6)
* fix ``base.py`` in resetting source config when no config is present for current harvest source