Add removeaddress RPC call (remove watch-only address)

The current master has the feature to add watch-only addresses (RPC `importaddress`). There is no way to remove watch-only addresses (looks like the unwritten law to not remove/erase anything from the wallet ;-). This PR adds it. The RPC call is named `removeaddress`. It has only one argument, the address/script to be removed.

You can't remove the address you don't watch in the wallet, you can't remove the address with a privkey.

Problems/questions:
1. After the call, your walletdb contains transactions added by previous (optional) rescan of the call `importaddress`. I do not touch them, because you can always run `bitcoind -zapwallettxes=1` to get rid of them.
2. Qt GUI needs a small fix in `TransactionFilterProxy::filterAcceptsRow()`:
```
if(address.isEmpty())
    return false;
```
(or similar...) to not display these transactions after watch-only addresses are removed.
3. Qt GUI needs to auto-refresh the list of Recent transactions (the right tab on the main screen) because after `removeaddress` the list is not updated (watch-only transactions are still shown). Or you can restart GUI...
4. I do not remove the address from addressbook, because it could have been added even before adding it in `importaddress`.