1.7: change currency three-letter-code to DOGE

also change length of currency amounts in UI. That enables you to send transactions with much more value through the gui, the current limitation was brought up in #134. Some check that the sent amount is below MAX_MONEY has to be added at some point since in dogecoin MAX_MONEY is lower than the coin supply.

Identical to inutoshi/inutoshi@8dcef8791a02327c8061235df7a77e2da6b59ee2