Added support for installing from the beta, canary and release channels

Added support for --ember_only and --ember_data_only
Added support for the beta, canary and release channel
Added tests for the ember:install generator
deprecated the use of --head on ember:install
changed the README.md to reflect the changes
Added a full test set for all options
Merge pull request #273 from hajee/install_named_releases

Added support for installing the beta and the canary release