[BUGFIX] error when trailing slash present in argument to generator

This PR provides a hook inside the Blueprint install function that by default throws and error when there is a trailing slash in the argument to `ember generate`.  This can be overridden within each individual blueprint by overriding the normalizeEntityName function inside the constructor for that blueprint.

Let me know what you think.

resolves #1164
Can you give a nice suggestion on how to fix the issue? Something like "You specified 'foo/', but you can't use a trailing slash as an entity name with generators. Please re-run the command with 'foo'." 

Or something that sounds even cooler than that...