Supporting :create_scopes with :prefix option

Hi Cassio!

If I have reserved word in my Enumeration (e.g. new). When I try to use both options, the `create_scopes` method tries to create scope with the reserved word and exception is launched.

Tks!
I followed `create_helper_methods` and `create_mutator_methods` for change this.
I would like to suggest a small refactor for you in another pull request. What do you think?