Coinbase Unit Tests & Some Docs

- Added the option to retrieve the 24 hour high/low from historical spot prices for the Ticker.
- Changed withdrawal to return the coinbase tx id instead of the blockchain tx hash so that it will be easy to retrieve further information including the tx hash.  Still torn on this one as I feel that most people using the generic interface would usually just want the blockchain tx hash.
- I didn't add historical prices to getTrades because it feels too misleading.
- Includes a couple of deserialization bug fixes caught from the new tests.