refactor add_and_commit method

move commands into helper methods
test add_and_commit
introduce new geogit::repository

used for adding and commiting to the geogit repo
refactor geogit add_and_commit to use new repository class
remove method parameter; not needed anymore
Merge pull request #5 from milafrerichs/refactor_add_and_comit

Refactor add_and_commit method