Some refactoring for future fixes

[anton-ryzhov]

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.

[reingart]

Thanks Anton!
I'm in the middle of a Google Summer of Code project so I don't have much time to review this.
If you think this is convenient, I'll merge it, but please it would be wise if you can take a look to ensure you're not introducing future issues (for example, overwriting internal instance attributes like self.soap_ns, that change look suspicious, and it can be confusing as it is already a __soap_ns private instance attribute).

[anton-ryzhov]

Good luck at GSC.

I would be more confident if unittests passed before and after changes) Now I only can compare errors before and after changes and ensure I haven't brought new.
I've double checked name collisions, but your remark is significant. I've renamed this property.

If you feel uncertainty about this fixes right now maybe we should wait until you'll have enough time.
I had a serious reason to rewrite entire _xml_tree_to_services on the next step (I'm preparing new PR). This definitely fixed critical bug for me, but there I'm not sure I've done everything correct. I'll need your to check it.

[reingart]

Yes, the unittests needs care, many webservices are no more available and others simply fails because it cannot download the WSDL attached to each ticket in googlecode.
I started to clean up the test but I hit some other serious issues to me, so I had to pospone it.
If you could finish the test's clean up so Travis CI don't keep failing, that would be great.

About you uncertain feelling, I prefer taking the risk of merging your PR if no serious regression is introduced.
I've to review the wsdl parsing code for my own projects too, to I'd prefer doing it with you changes commited, that will be more productive and your refactory will make easier to fix the outstanding issues.

Thanks again!

[anton-ryzhov]

Okey. Merge this now, I'll introduce my rewritten version based on this PR and we'll work on it to fit library to ours requirements.

Test fixes — sorry, I don't have enough time for it right now.