Database Streamers

Database streamers allow us to easy stream over text stored in databases (remote or otherwise) and apply the common streamer methods to them (ie convert them to vw format, sparse matrix format, or otherwise deal with them document by document).  

Main additions:
- Database streamer abstract class
- Two implementations added, MySQLStreamer and MongoStreamer
- Add .to_vw() method to the BaseStreamer to clean things up a bit
- Streamer test setup