Make gem working for ActiveRecord 3

Use .scoped method for AR3.2 and .all for AR4
Replace `scope :cursor` by `def self.cursor` (because you can't get previous
scope in ActiveRecord 3, when use `scope :cursor`)
Downgrade version of database_cleaner

Because >1.1.0 versions raise error without pg gem
Increase minimal version for ActiveRecord to 3.1

It doesn't pass tests on 3.0 version
Upgrade version to 0.0.2