make it possible to configure the content type of a github webhook

(Depends on (and includes) https://github.com/ansible/ansible-modules-extras/pull/8 as I wasn't able to test it without fixing shadowing imports first)

Adds `content_type` argument to `github_hooks` module to allow user to choose between json and form encoding for requests from github.  Default is kept as json so it should not break backwards compatibility.

The patch changes the signature of `create` so that it no longer matches the other commands in the module.  I also haven't spent too much thought about argument ordering.  I'm happy to try to make things match if you are unhappy with the style.