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

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

All tests on all versions of ruby working now.

Cut back on testing time by testing on a testing condition before fetching the ember.js files

Better test for Install Generator

Use a real rescue to assert the error's
Got rid of the error messages
Using VCR to mock the fetching of the files

Changed release option to channel

Deprecated --head option
Changed release option to channel
Added channel release
changed the text in the README.md to reflect the changes