Moved *all* the matchers from RSpecApi

* Added include_content_type matcher
* Added have_prev_page_link matcher
* Added be_valid_json matcher
* Added be_a_jsonp matcher
* Added be_sorted matcher
* Added be_filtered matcher
* Added have_attributes matcher
* Added run_if, to execute matchers conditionally
* Make sorted/filtered matchers pending if given an empty array