Adding support for sell/buy at market price - Adding support for all markets

I was previously unable to pass sell or buy orders at market price. It was due to a conversion error between the python dictionary and the json object when generating the hash. The python None was converted to the string "None" instead of an empty string.

I also added the support for all markets in the API. You can specify market="ltccny" to pass an order on the ltccny market for instance.