rename list to list_ to avoid shadowing in github_hooks module

The definition was leaking into ansible.module_utils.basic and causing
type checking to fail when running module as script.  Not entirely clear
why this should be the case.
make it possible to configure the content type of a github webhook
mark list as private instead of shadowing
mark all actions as private
Merge pull request #9 from bwhmather/feature/github_content_type

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