Add a git clone strategy based on archive tarballs

Depending on git repo size, using the archive api can result in speed improvements due to the way git works. 

Some things to note:

- This provides a shallow clone of the repo.

- Submodules and other git operations will not work because no .git directory exists

- For private repos an oauth_token is required in the payload from travis-hub.

- I still need to add support for hub to return either an oauth token if the repo is private, or a default token so Travis is not hit by GitHub API limits (similar to composer)

This is good to merge in and for us to test on Org.