paymentserver: style fixes / cleanup

- rename reportError() into message() to be in line with our default
  message() signal/slot naming (and can be used for all types of messages)
- rename some QStrings to not collide with message() function
- add a missing message for malformed URIs that IS also used in BitcoinGUI
- fix / extend some comments and misc style fixes
Merge pull request #3108 from Diapolo/paymentserver_style

paymentserver: style fixes / cleanup
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.