Update GitRepo to use pygit2

[splattater]

This commit changes the git library from GitPython to pygit2.
With this commit the installation of QuitStore changes and libgit2 is needed to fulfill the requirements of pygit2.

[coveralls]

Coverage increased (+0.1%) to 13.391% when pulling 0fcec64 on feature/pygit2 into 35e08b7 on master.

[white-gecko]

The quit.log is also commited and thus my verification script fails.

[coveralls]

Coverage increased (+0.2%) to 13.46% when pulling a085742 on feature/pygit2 into 35e08b7 on master.

[splattater]

I added an option to enable "pathspec" in pygit2 when using the index_add_all method.
The method will be called with a list containing all files that are configured in config.ttl, so every unknown/untracked files won't be added to index.
You could also update your gitignore to avoid that issue.

[white-gecko]

With --pathspec option it creates empty commits.

[coveralls]

Coverage increased (+0.2%) to 13.448% when pulling 097f2f0 on feature/pygit2 into 35e08b7 on master.

[white-gecko]

https://github.com/AKSW/QuitEval/blob/master/verify.py: OK