Adding optional support of --depth for github only.

Hey guys, 

here's my implementation of detection of --depth support.

Related issue: https://github.com/CocoaPods/CocoaPods/issues/2481

For now it's only detected by URL github host name. If it's not github.com we don't allow usage of --depth param.

I didn't know how to fix the tests. I would appreciate someone from the project to help me out write those.

Thank you, Ladislav