Some refactoring for future fixes

Hello guys! I'd chosen your library as best available for python and I've started to use it in my project.
I've faced with some issues, and going to fix it and make pull-requests.
As first step I've made some refactoring — today `wsdl_parse` is too complex to understand and work with.
Looks like I've made no degradation with this fix according to tests.

I hope you'll like it and further fixes.