Implement xml.Unmarshaller for SOAPBody

[md5]

I'm opening this PR to get some feedback since I'm very new at writing Go code.

These changes allow unmarshalling and marshalling to happen in a single pass, which allows a broader range of SOAP responses to be parsed. In particular, it handles the case where the xmlns used on the <SOAP-ENV:Body> entries is declared on a parent element instead of on the entry itself.

I would have liked to add some tests here, but I'm not clear on how to structure tests that use the generated code from soap_tmpl.go. I'm happy to add some tests if someone can give me some pointers on how to do so. I was able to run make test, so I at least don't think I broke any of the existing tests.

Fixes #37

[md5]

BTW, I used my modified version of gowsdl to start working on a client for VirtualBox here: https://github.com/appropriate/go-virtualboxclient

[c4milo]

This looks good to me @md5. Thank you!

[md5]

Thanks @c4milo 👍

I'm still willing to add some tests in this area, assuming I can get a start in the right direction. Perhaps @sanbornm might have some ideas here.

[sanbornm]

@md5 I think we need to figure out how to check if our generated code would compile or not. We are currently testing it by running it through go fmt (https://github.com/hooklift/gowsdl/blob/master/gowsdl_test.go#L56) which catches a lot of errors but I am not sure it checks everything.