Add confirmed balance for address/worker API call

This is hopefully a good starting point for a simple API function that allows a worker to check their confirmed balance. This might have performance problems, since it opens a new redis connection each call, but I'm not good enough at node to really know of a good way around it. 

The intention is to add an unconfirmed balance field as well, but this is a good start. 

Usage: `/api/balance?coin=Testcoin&address=qC7i5QD1FRjJgjyMesU6eSFbakcgAvfFCQ`

returns:

    {"confirmed": 1234}