Added support for installing the beta and the canary release

This should fix the problems with #271 & #272. I've given it some needed attention. I added tests. 

It still has some issues:
* The tests take *forever*. This is because it really fetches the files from github
* Although the error situations are checked and catched. The generator still displays the error messages. I think this is distracting, but don't know how to fix it
* The tests don't check for the exception message. I don't now how Thor manages the exceptions

Suggestions for the future
* deprecate the --head in favor of --release=head
* Add --release=release after ember-data has been released. This will install the released versions.

Regards,

Bert