filtering "mousemove" for changed values of X or Y

"Ghost" mousemove events where mouse location coordinates X and Y do not vary would fake user activity.

They occur in Chrome browser on Windows when the mouse pointer rests over the test page with *ifvisible.js* on it while another bowser window reloads its document form another url.
In the other browser window simply issue
```javascript
setTimeout(function(){ location.href = "https://www.google.de"; }, 10000);
```
, rest the mouse pointer over the *ifvisible.js* test page and observe fake activity after 10 seconds (when the other window reloads).

Since I do not know Coffeescript I simply added the mouse position filter in the .js file.

Nice library!
Best regards
Matthias Seemann