Prefix database tables

Adds "double_entry_" prefix to all the tables.

Open to a different approach because I'm not sure I love this public class method.  We could just specify the table name in the individual classes, or have them inherit from another class (like DoubleEntry::Record < ActiveRecord::Base, which defines the prefix).
not sure how i feel about this. This is so that we can provide a hint as to how this will be integrated into a larger app, and we are assuming that the larger app has users. Should we not prefix this?