Added extensibility points

This commit is refactor only (there is no behavior changes and no new features added). We've added some extensibility points to have a possibility to override some methods in our classes generated by http://EasyWSDL.com generator.
Additionally we started to work on MTOM transfer (SOAP with attachments) and those extensibility points allow us inject a new features into ksoap2 library.

We've already tested this commit and all our tests still works good (so existing behavior of ksoap2 hasn't been changed).
It would be nice if you create a new release with these changes