Add faceted search

Added sunspot_rails, sunspot_solr in order to implement the faceted search functionality.
Added the faceted search (filters) for: Brand, Carriers, Conditions. Price range is still pending.
Added Fabrication and Faker gems for easier ads creation.
Added a search form in the index view where users can search in for words in the ad title and description
The 'fake' ads are defined in the seed.rb file.
Added link to navigation to the new_ad_path
Added dynamical user information in the ads/show

IMP!! Once merge please follow:
1. bundle install
2. rails generate sunspot_rails:install 
3. bundle exec rake sunspot:solr:start  (Also run this command if  you  ever get this error Errno:: ECONNREFUSED (Connection refused - connect (2)) ”)
4. bundle exec rake db:reset   (To create the users and also a whole bunch of fake ads that already contain attributes to be filtered)