Add separate bitcoin-rpc client

This adds an executable `bitcoin-rpc` that only serves as a Bitcoin RPC
client.
The commit does not remove RPC functionality from the `bitcoind` yet,
this functionality should be deprecated but is left for a later version
to give users some time to switch.
Merge pull request #3082 from laanwj/2013_10_rpccli3

Add separate bitcoin-rpc client