New generator to install emberjs beta build

This is not ready for merge. It is meant to get feedback.

The idea is to have 

       rails generate ember:beta   --head
       rails generate ember:stable  --head


This is necessitated by  Yehuda's [blog post](http://emberjs.com/blog/2013/09/06/new-ember-release-process.html), that oultlines emberjs release process. Since the the current **ember:install --head** builds from master, which is more or less equivalent to the **canary build**, that was referred to in the blog post. I felt it will be a good option for have install for the other builds ie the **beta**  build.

I asked Yehuda on the #emebr-dev #irc channel and said this will make sense.

The current problem is that when I point to the **beta** or **stable branch** of  **emberjs**, it only fetches **ember-data beta-1** instead of **beta-2** and it does not even fetch **emberjs**