More scaffolding

This pull request asserts a couple of opinions, including...

* major
  - using `libgit2` and `git2go` for future `git` operations (asserting that this is the correct course of action over using `exec.Cmd`, as it will be much faster, albeit slightly more involved to compile for releases)
* minor
  - `Makefile` style
  - not installing with `gem` (update in documentation)
  - using `ginkgo` as the testing library of choice

request comments.

**edit:**

Additional assertions:

* major
  - there will be two executables, `git-duet` and `git-solo`
  - the top level package for shared libraries will be called `lib`
* minor
  - *(none)*
Given how expensive a `go get` is, even when it's a no-op, I really don't like having this as a phony target.  This is why I do things like touching placeholder files so that make skips over completed one-time tasks.  I want my feedback loop to be under 3 seconds if at all possible :smiley_cat: