Support for mercurial

[turicas]

Everything we use that is related to Git is inside pyquality/git_utils.py, a really small and simple file.
To add support to Mercurial we should create a file like this, replacing Git commands with Mercurial's and then creating a way to discover if the repository is Mercurial or Git to decide which module (git_utils.py or hg_utils.py) to use.