Update Dulwich, remove duplicate code

Update to a newer version of Dulwich, and use functionality from upstream Dulwich where possible. 

This allows dropping of the copy of configobj that rabbitvcs was shipping.
@jelmer and @CloCkWeRX just a note that in Ubuntu 14.04 only dulwich 0.9.4 (called `python-dulwich`) is available. Not sure if the solution is to downgrade this requirement, add dulwich 0.9.7 to RabbitVCS PPA or something else?