Delegate 'to_json' and 'as_json' methods to the actual attribute value.

This fixes a circular reference error from active support when a Rails model
with an enum attribute is converted to json.
Moved 'clear_user' helper method to spec helper file, so other spec files can use it.
Ensure 'to_json' method works properly on a model with an enum attribute.
initial commit
bump to v0.0.5
Extend 'RailsAttrEnum' and add 'attr_enum' on 'role' attribute
Ensure the table for the model exists before trying to check column_names

I discovered this as a bug when I tried to freshly load a development
db on a Rails app (`rake db:schema:load`), and the call to
'column_names' in 'add_attr_enum' was crashing the rake task.