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!
If I understood correctly, you are using the `:prefix` of the `:create_helpers` option. I really think that `:create_helpers` and `:create_scopes` are very different things, so each one of them should accept suboptions. Something like 

```ruby
has_enumeration_for :foobar, :create_scopes => { :prefix => true }
```

would be better.