Rails 4 support

* Drops support for Ruby 1.8
* Drops support for Rails prior to 4.0
* Misc style updates for newer AR scopes/arel
* Use ActiveSupport::Concern
* Remove the association scopes
* Remove the datetime scopes
* Remove the duration scopes
* Remove the ranking scopes
* Remove the numeric scopes
* Remove all of the search scopes, except for exact match
* Remove the boolean "count" scope