Storage backend implementation using git

[trevex]

The basic storage interface is outlined here:
https://github.com/kubism/smorgasbord/blob/master/pkg/storage/storage.go

The git implementation was started but has to be finished:
https://github.com/kubism/smorgasbord/blob/master/pkg/storage/git/git.go

The save-routine will be most critical the core idea is that multiple replicas of the server are possible as changes (commits) will conflict and will have to be reapplied (essentially compare and swap).

Implement the git storage backend and test it.