WFS updates

In this pull request, I'm going to collect changes in the WFS code. The code should be tested carefully on running servers. Most of existing tests are not working any more, because of servers are down. I've add two tests with offline mode testing.

There is new WebFeatureService_ class (not sure about proper name), which is base class for WebFeatureService_2_0_0 and WebFeatureService_1_0_0. It contains some common methods. Should be applyed to WebFeatureService_1_0_0 as well (not sure about any running server around).

Please, do not close this pull request, after everybody agrees, changes are fine. I'll continue working on it.