[1685] Move from Metasearch to Ransack

This PR is for #1685, and is a continuation of the work @pavelbrylov did in #1811.

#### BTW
+ this is on @gregbell's repo
+ be sure to add an extra set of quote marks when referencing this branch name: `'"metasearch->ransack"'`

#### Using this with Rails 4
This doesn't currently work 100% with RaIls 4, but it's close. You need to specify these gems:
```ruby
gem 'activeadmin',         github: 'gregbell/active_admin', branch: 'rails4'
gem 'ransack',             github: 'ernie/ransack',         branch: 'rails-4'
gem 'inherited_resources', github: 'josevalim/inherited_resources'
gem 'sass-rails', '~> 4.0.0.rc2'
```