Port of wallet.sh regression test to Python, and minor edits to util.py


Also included are two edits to util.py: 

1) Added function to initialize datadirs with an empty blockchain (with a specified number of nodes, but no mined blocks.) 
2) Added a parameter to gather_inputs to allow the caller to specify the number of confirmations required (default = 1). 

Finally, I deleted wallet.sh and updated rpc-tests.sh to run wallet.py instead of wallet.sh.