Add ability to download and install chef from GitHub.

This replaces https://github.com/test-kitchen/test-kitchen/pull/473, with some minor changes:

* Uses the tag `:github`, instead of `:chef_git_url`, and set `:github "<user>/<repo>"`
* Requires `:branch` key if `:github` key is used. `:branch` can be set to a branch name or commit sha.
* Overrides `:require_chef_omnibus` to `true` when downloading GitHub source code, if set to `false` or `nil`
* Will fail immediately if the source code cannot be downloaded from the generated url or if the chef gem cannot be built.

Works on Ubuntu 12.04 on both vagrant/virtualbox and ec2. Works on CentOS on vagrant/virtualbox and I'm pretty sure ec2, too.

@fnichol, @sethvargo, @sersut, @lamont-granquist, @adamedx, @danielsdeleo
Umm probably not