adds new rake task to add twitter & github usernames to api

The social data is based on a manually-created hash containing twitter
and github usernames for each 2013 fellow. "fellows_social_hash.rb"
creates "fellows_social_data.json" from that hash. The rake task then
uses the data from the json file to update the db.

The rake task has been modified so that you can run both the :fellows
and :social tasks by running `rake load_data`

Additional changes:
added "pry" to Gemfile to allow cfa_fellow_scraper.rb to run

updated cfa_fellow_scraper.rb to grab fellow name from the displayed
text rather than the name attribute (which resulted in misspelled names
in some cases)

updated README with instructions for loading or resetting the db
depending on whether or not you already had created the db
Clean up some of the intermediary migrations
Add new schema back