gitapi module => restfulgit package

* Switches from a module to a package
 * More in line with other projects
 * We'll need to do this anyway when we go multi-module (e.g. for `/commits/` support)
* Names that package `restfulgit` for consistency
* Also enables [Coveralls code coverage tracking.](https://coveralls.io/r/hulu/restfulgit) (We're currently at 89%)

/cc @dlindquist @rajivm for code review