Fix gemspec warning
Add Gemfile.lock to .gitignore (best practice for gems--see http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/)
Refactor evaluate_ methods to increase code readability
- Remove ActiveSupport from gemspec / spec helper, since Mongoid already depends on it
- Specs: Rename stub! to stub in keeping with latest RSpec release
Add :model_name option which allows custom override of model name