Slaves should pull git repos from the master

- This changes our original design of passing the
project type parameters down to the slaves verbatim.
We now modify them so the url points to the master's
Git repo
Add tests, respond to review feedback, respect shallow param
Check for 'shallow' build argument correctly
Add CLI argument for 'shallow' to build command
Merge pull request #43 from box/slaves_use_master_as_origin

Slaves should pull git repos from the master
If the url to the repo has a colon in it, remove it.

- Also separate out constructor functionality into its own method in
  git.py