Custom Extractor for clinicaltrials.gov (#305)

* Add prototype of custom extractor for clinicaltrials.gov

* Add .DS_Store to gitignore

* Make tests for title, author and date_published selectors pass

* Make content selector test pass

* Fix date_published test

* Rebuild

* Remove .DS-Store from gitignore

* Improve extractor and text/fixture of clinicaltrials.gov

Author: kennyle3377