Add 'Clone' menu option to the Repository viewer activity

Hi there - this patch provides some nice integration with Agit (the Git client app) which has support for doing full clones of git repos onto the SD card, but no GitHub integration.

The 'Clone' menu option launches the `org.openintents.git.clone.PREPARE` intent which will give the user a chance to review the details of the checkout (target dir, bare/not-bare, etc) before starting the clone proper. The menu option is hidden if no app can be found to handle the intent (e.g. if Agit isn't installed).

We have to pass the source-uri for doing the clone, but the GitHub api doesn't provide the clone url - only the project url, so we work it out from that, using an ssh url if the repo is private.

I have to apologise but unfortunately I wasn't able to get Gh4a to compile in the time I had available (I couldn't find the mygson dependency for https://github.com/slapperwan/github-java-sdk ) so I've been coding blind to some extent, but I've already tested very similar code against Hubroid, so I think it should work just fine - please let me know if there are any problems!

best regards,
Roberto Tyley