Remove ignored columns from attribute_names

I ran into a problem with associations which uses the attributes_names instance methods and resulted in a lots of deprecation warnings from ActiveRecord.

This patch fixes the error by applying ignored_column? logic to attribute names. I refactored slightly to use ActiveSupport::Concern since the module now mixes both instance and class methods.

I also added an rspec test suite and Rakefile and bumped the version to 0.2.0 in the gemspec.

One other thing that is missing is a license file. Could you please add one to make the world's lawyers happy :-)