alias -> alias_method

This addresses the closed issue #131 basically alias and alias_method work very differently in terms of scoping you can read more [here](http://blog.bigbinary.com/2012/01/08/alias-vs-alias-method.html) essentially because of this `custom_build_access_token` was never getting called and the Hybrid Authentication flow described in the readme was impossible. Now the `custom_build_access_token` gets called as expected.