Add enum_name! methods to lib/acts_as_enum.rb

For convenience, I added enum_name! methods to lib/acts_as_enum.rb. for example:
```
class User < ActiveRecord::Base
  acts_as_enum :status, :in => [ ['disable', '0', '冻结'], ['enable', '1', '激活'] ]
end

user = User.first
```
the `user` now has methods: `enable!` and `disable!`
`user.enable! ` # will update user.status to 1
`user.disable!`  # will update user.status to 0