Implementation of the providers for git and github

This pull request contains the implementations for the git and the github provider. It contains also the client- and serverside handling for the new api routes with the informations about the github user and repository.

The implementation of the github provider is not complete. There are some things that can be improved. For instance the support for subdirectories. Even the support of static files is actually not implemented.

I know that this pull request is a little to large ;-)