[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
Absolutely.  I added the ui param before I realized that there is a catch that prints errors through ui in red.  Fixed!  

Good catch! :)