Handle github hook authentication

[julienvey]

This PR handles github authentication for webhooks.

See developer doc https://developer.github.com/webhooks/

[jawher]

Nice work !

A couple of remarks:

• Why overload the authentication middleware to handle Github specific authentication ? why not just create a Github authenticator middleware and apply it to the github route only (which should let you use mux.Vars to extract the project id) ?
• Due to the previous remark, I think we should split the server endpoints into 3 sub-routers: bazooka, github and bitbucket, each with a specific authenticator. I'm not sure about this one mind you.
• What's the purpose of making GithubAuthenticator validation logic configurable (a field in a struct) and implementing it in context.go ? shouldn't the Github specific logic be fully contained in github.go ?

[jawher]

LGTM