Adds `content_finder` method to hide away Nokogiri

The implementation of this Gem shouldn't require as much knowledge of how Nokogiri parsing works. The `get_listing` method is deprecated in favor of `content_finder` (same name as the online utility on the SODA XML Team console). This new method returns an array of document objects that are easier to work with.
Fix for relying on an unneeded library.
Merge pull request #6 from seatshare/feature/simplify-listings

Adds `content_finder` method to hide away Nokogiri