Make gopkg usable as a package itself

This PR makes gopkg usable as a package itself. It moves the literal `gopkg` server command to `cmd/gopkg`.

For an overview of how the package works you can review the documentation at http://godoc.org/github.com/slimsag/gopkg right now.

@niemeyer Being 17 commits (with the last few being a bit large) I understand if you don't want to merge this. I think the code I've committed is high quality and has nice idea's behind it though.

If you are not comfortable with this then I will keep the repository around as an open proposal to gopkg and I am willing to work with you/others here at a slower pace to integrate such a feature (even if the implementation is not exactly this).

Cheers!