Add API view and Twitter streaming support for running Battles

When the new api-battle-start API endpoint is called, a new Battle is
started, using the Tweepy python library for the Twitter streaming
API.  A new "Battle thread" is initialized and started to run in the
background, processing the Twitter stream, tracking the counts for
each of the Players (hashtags/keywords) and updating the database
accordingly.  Once the battle length has been reached, the stream
processing exits and the battle ends.