Add support for build pack branches

Add support for build pack branches
Fix test case to support proper URIs

Add Shell escaping back

Move git commands to its own class

Add integration tests

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>
Auto-merge remote-tracking branch 'upstream/master'