refactor(combat): move combat code into its own directory to decouple from game

The combat system has grown to be complex enough to warran its own top-level directory.
- Combat has a complex state machine that confuses the game directory.
- Considering moving combat into Angular almost entirely.
 - Test performance of angular in realtime game system.
 - Fewer awkward Backbone.Event triggers between game and UI code.
docs(commits): outline commit convention in README.md