Add progress bar to gitlearn

Gitlearn has two stages: 1) parse git logs and 2) create the training database. This adds a progress bar to (2) and a status message for (1).

    tavish@temeraire:~/code/ipython (master)$ gitlearn
    Parsing git logs... Done.
    Creating training database at .git/coaching-data.sqlite
    [################################] 12381/12381

Closes #15