Advanced search enhancements & Autospec gem

The project proposes new filters to advanced search and extraction of the rake task for running unit tests into a separate gem. Advanced search enhancements contains 1) searching posts containing all of the entered tags and 2) searching posts with images (with extension for searching by any filetype). The benefit for the Discourse community is better filtering of posts with emphasis on the search performance which will be useful for searching in bigger forums. By extracting custom way of running unit tests into a gem, we separate concerns as the tests running library is not dependent on the Discourse, Discourse code base gets smaller and it will be beneficial to whole Rails community.