Create API for commits and files

[phyrog]

Needed routes:

/repositories/:id/commits # List of commits in that repository
/repositories/:id/commits/:hash # One specific commit
/repositories/:id/commits/:hash/files # List of files at one specific commit
/repositories/:id/commits/:hash/files/:file_path # One specific file at one specific commit

Repository should include relationship to the list of commits. In the list of commits each commit should have a relationship to the list of files and each file in this list should link to the file url.
The repository should also include some information about the branches of the repository, especially which one is the default branch (needs to be implemented in the models first, see ontohub/ontohub-models#52).

[phyrog]

@eugenk works on this in #9.