packages recipe, SCM requirements, README/tests updates

This pull request represents a slight reorganization of the cookbook to help limit unintended installation of extra system packages and provide a configuration-based method for installing Go packages.

* Removed the `requirements` recipe as the SCM packages are not actually required to install or run Go and may be unwanted in certain cases.
* Removed the `install` recipe and moved that functionality to the `default` recipe.
* Added a `packages` recipe that allows users to specify packages to install via a node attribute.
* Added SCM installation as an option controlled by a node attribute.
* Added support for Mercurial.
* Added a LICENSE file and created consistent recipe headers.
* Updated default Go version to 1.1.2.
* Updated Gem dependencies.
* Updated the README.

All of the tests are passing.

Should you decide to merge the request, the differences between this code and the existing version probably warrants bumping the version number to 1.1.0.

Thanks!