Add Batman.InclusionValidator

Mimic's Rails' inclusion validator

http://guides.rubyonrails.org/active_record_validations_callbacks.html#inclusion
InclusionValidator and ExclusionValidator in the fashion of Rails.
Rename variables for readability and to avoid confusion
exclusion, not exlusion
Docs for inclusion/exclusion validators
Add tests for new validators.
Make changes from @nickjs' feedback.
Merge pull request #668 from danott/validators

InclusionValidator and ExclusionValidator