Add VCS Integration

[theory]

The plan needs subclasses (or roles?) to support VCS integration. This would included:

• The ability to find source code from the VCS history (change name and tag)
• The ability to read such source code files and pass them to a client. Might need to have them as real files
• The ability to read the VCS history and generate a plan from it, for bootstrapping (#24)
• The ability to add tags to the VCS history via sqitch tag and other automations.

And no doubt other stuff.

[theory]

Git has now been explicitly integrated into the checkout command. The next step should be to abstract it into a VCS-independent interface. I'm thinking it should be called App::Sqitch::VCS, and when you instantiate it, it automatically detects which VCS you're using and loads the proper class (factory constructor). The interface for such a class will be defined by a role that requires all the appropriate methods. We would start with those required for checkout and add more as we need them.