Added support for coinbase. See pull req for more documentation.

I added support for coinbase. Coinbase doesn't have a unified ticker URL, so I modified the TickerUrl stuff to handle an array of ticker urls. I preserved the old interface, but you can also include an array of strings and they'll all be fetched. The JSON object returned will look like: {"url0": {data from first url}, "url1": {data from second url}}. If only one URL is used, the old object will be returned (e.g. {"amount": 123, "currency": "USD"}).