Fix json bodies

Fixes #49 

Seems that params is just the wrong attribute to use ([docs](http://docs.pylonsproject.org/docs/pyramid/en/1.5-branch/api/request.html#pyramid.request.Request.params))! 

Added a new test to cover this use case and renamed the existing acceptance test modules to better reflect their contents.
Yes, this tests fails on the old code and succeeds on the new code. It is extremely dependent on your HTTP library because it has everything to do with the precise details of your request.

You'll notice I made this call `.post` not `.post_json`. The difference there is that all the `_json` methods in WebTest force a content type of "application/json" ([reference](https://github.com/Pylons/webtest/blob/master/webtest/utils.py#L22)). On Pyramid's side, [self.params](https://github.com/Pylons/webob/blob/master/webob/request.py#L851) combines .GET and [.POST](https://github.com/Pylons/webob/blob/master/webob/request.py#L762) and the latter is (IMO weirdly) scoped to only return form-encoded parameters.

So in summary, this is all about Content Types.