Blacklist API

Addresses #496.

This PR adds two new API commands: `addAddressToBlackWhiteList` and `removeAddressFromBlackWhiteList`, which add and remove black-/white-list entries (respectively) depending on which of `(blacklist|whitelist)` the user has set.

Two new API codes (28 and 29) are added for alerting API clients of when an entry already exists or does not exist (also respectively).

In `bitmessageqt/__init__.py` `loadBlackWhiteList` is refactored into `rerenderBlackWhiteList` to be more consistent with the rest of the rerender methods. Finally, it also accepts a new UI signal to refresh the black-/white-list UI pane.

There was no testing suite to speak of, but I did test the breadth of the functionality manually as thoroughly as I could.