Added options to ember:install beta or canary version of ember & ember-data

I would like an easy way to be able to use a canary of beta release of ember in my rails project. Right now ember-rails only supports `--head` options to `rails generate ember:install`. This pull request adds the options to specify a release

 ```sh
rails generate ember:install --release=canary
````
installs the current canary release

 ```sh
rails generate ember:install --release=beta
````
installs the current beta release.

For backwards compatibility, the --head option is preserved.

Current code could use some love. I'm not really into Thor.  and the check's for the right options is probably not as it should be.