Re-modularizing 3.0.x

This is still a work in progress, but I'd like to get the first part pushed so that others can contribute on top.  Themes:

1. Removed almost all duplication of exceptions and moved the com.mongodb ones into driver core
2. Simplified CommandResult and removed dependencies on it from exceptions. 
3. Started removing other duplication, including ServerAddress, ReadPreference, WriteConcern, and MongoCredential. Moved com.mongodb ones into driver core.
4. Renamed org.mongodb.<package packages to com.mongodb.<package>.  The org.mongodb package itself is the only one left.