RPC command "eraseprivkey" for removing a specified private key

- Now you can instantly remove a specified private key
- No need to rescan for wallet transaction in case you use "importprivkey", just enter:
   importprivkey <bitcoinprivkey> [label] false
   and you would have your wallet ready again in 1 second
- If wallet is encrypted, it would be necessary to use walletpassphrase in advance (as it's same rule for importprivkey)
- If watch-only pulls get merged, it would hopefully help to add 'sweep
private key' functionality (like Armory Bitcoin client)