Add a test for invalid PUTs which do not have a valid auth_token
Implement a working merge controller

Example usage via curl:

    # merge branch leto_study_1003 to master
    curl -X POST http://localhost:8000/api/merge/v1/leto_study_1003?auth_token=$GITHUB_OAUTH_TOKEN

    # Be explicit about base branch to merge into, i.e. merge leto_study_12 into master
    curl -X POST http://localhost:8000/api/merge/v1/leto_study_12/master?auth_token=$GITHUB_OAUTH_TOKEN