Show rescan information on executing importprivkey

[ajochems]

When a user executes an importprivkey command a rescan will occur. However to the user it seems that the wallet 'hangs' and might kill the wallet application causing a possible corruption of the blockchain. This can be prevented by giving the user information about the result of the importprivkey command and a possible rescan of the blockchain.

[Bitraise]

Created branch issue-4 pulled from master and added some messages during rescan. If ok, please merge

[ajochems]

Let me build and check and will merge if ok, thx

[Bitraise]

It didn't work, built it myself in a new dev enviroment. It writes the messages to log and not to the console. Progress should not be shown imo. its a rpc call which returns a single response.