Major updates, breaks backwards compatibility

It might be necessary to change some of what I've done, especially the parts that break backwards compatibility. Or this version could be considered a new major revision. In short, I have:
- Replaced the tuples getting tossed around in Arbitrer with Trade objects.
- Broke out most of the functionality formerly in Arbitrer into the Market, MarketChain, and TradeChain classes.
- Changed all the private_market classes so they now take and execute Trade objects.
- Removed the dependence on USD as a pivot currency, allowing users to choose an arbitrary pivot via config.py.
- Added the option of defining a config.json file that overrides config.py and automatically gets reloaded when changed.
- Added dependencies on Watchdog and Blinker (for the line item directly above).
- Split tests into unit_tests and integration_tests.
- Added a BTC-e private market class for automated trading.