Merge pull request #2 from holyketzer/master

Rails way refactoring. Fix gemspec issue. Add fetching listing by sku
Merge pull request #3 from holyketzer/master

Add loading listings for products#index
Merge pull request #4 from holyketzer/master

Add fetching listing by id. Fetch listing for order#populate action
handling spaces when converting CamelCase to camel_case keys.
Added api methods to retrieve delivery options.
Fixing shipping methods api call.