Merge pull request #100 from mrjlynch/master

Update README.md
Updated Rubocop to 0.21.0.
Fixed compatibility with using default_scope.
Fixed compatibility with Mongoid 4.x, which removed attr_accessible of protected attributes.

See https://github.com/mongoid/mongoid/issues/3664 and https://github.com/mongoid/mongoid/issues/3663.
Upgraded to RSpec 3.0.
Added support for polymorphic relationship scope via inverse relationship or multiple scopes.
Added a CONTRIBUTING doc.
Updated mongoid version support in README.
Preparing for 0.4.2 release, compatible with Mongoid4.
Added bundler tasks, rake release.
Preparing for next development iteration, 0.4.3.
Scope method added
Fix scope reference on history tracks criteria
Merge pull request #110 from adbeelitamar/master

Fix scope reference on history tracks criteria
Preparing for release, 0.4.3.
Preparing for development, 0.4.4.
Adding 'reload' to trackables specs
Fixing conflicts