fetching emberjs builds via Curl 

This pull request use **curl** to fetch builds from **http://emberjs.com/builds/**
 rails generate ember:stable  --head still points to master

While the **stable**, **beta** and **canary** builds can be gotten with:

             rails generate ember:stable  --head
             rails generate ember:beta   --head
             rails generate ember:canary  --head
    
I hope this merges cleanly, if not I can try again but you can also clone to your machine to see why.