add Active Record scope only if scope option is passed

Use scope option to specify Active Record scope name.
Fix to use `write_attribute`

In rails 4.0.0.beta1, ActiveRecord uses `@attributes`
to keep its attributes (instead of instance variable named as attribute
name).
Even if instance variable is assigned, attributes is not updated against my
expectation.

So I use `write_attribute` to update model's attributes.
raise ArgumentError when enumerized attribute is already defined
extract Enumerize::Module class
build predicates on enumerize module
support AR scopes when attr is defined in a module
Merge pull request #55 from brainspec/module-scopes

support AR scopes when attr is defined in a module
return correct default value for enumerized attribute

when using `default_scope` with generated scope
closes #58
rails 4 support
Merge remote-tracking branch 'tricknotes/write_attribute' into rails4
update travis config
Merge branch 'rails4'
support for RailsAdmin

If RailsAdmin module is found, generate the #{name}_enum method that it
wants to see.

Related documentation at:
https://github.com/sferik/rails_admin/wiki/Enumeration
move defining method for RailsAdmin integration to integrations
Merge pull request #59 from brainspec/rails-admin

RailsAdmin integration
use default value from db column
Add :only and :except options to Attribute#options
make :only and :except options work with not arrays
update CHANGELOG
Merge remote-tracking branch 'origin/pr-62'