improve the wsdl parser to know about messages

[rubiii]

now that all options are gathered in one (two) distinct places, it's obvious that savon has too many options to work with a somewhat "leaky abstraction". i think we can do better.

currently, savon depends on the global :convert_request_keys_to option to generate the message tag for each request. i don't know if it's possible to help with all xs types (arguments), but we can certainly get the correct message name.

i started working on wasabi's sax-parser branch again and everything looks promising so far. i'll update this issue if there's any progress.

[rubiii]

this will eventually be solved by version 3.