added ability to manually remove active keys (re #63)

Now active keys will be removed the only when meta key is pressed up ( #19  ). But in this case we catch a bug #63.

I found the easy solution: manually remove handled active keys. For example:

```js
kb.on('cmd + c', copy);
kb.on('cmd + v', paste);

var copy = function () {
  // do copy
  kb.removeActiveKey('c');
};

var paste = function () {
  // do paste
  kb.removeActiveKey('v');
};
```

Of course, it's not very user friendly solution, but it's tiny and it's works just now. Can you add the PR as temporary patch?