Use update_column and where.not

Fixes #56

use a GuardClause

Adding Gemfile.lock to .gitignore. According to [Yehuda's post](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/):

Fixes ActiveRecord 4.0 warning

More info about imgration on readme

Removed rails 2 from readme

Documentation clarification

If you care about having objects frozen after calling destroy/delete, then please call freeze on that object manually.

Fix compability with rails 4.0 wrt to run_destroy_callbacks deprecation

fix alias methods for Ruby 2 compatibility

Add test coverage for has_many and has_many :through behavior.

Fixes #27

Fixes #32

Update_attribute is gone in Rails 4, add update_column support.