Fixes and tests for Protocol.windowHandlePosition

Current Protocol.windowHandlePosition implementation doesn't work in cases when we don't specify tabid (assuming current window).

Examples:
*    <code>client.windowHandlePosition(function(err, res) {
        console.log(res.value);
});</code>
<p>expecting: object with x and y cordinates of current window</p>
<p>receiving: TypeError: string is not a function</p>
*     <code>client.windowHandlePosition({x: 600, y: 100});</code>
<p>expecting: current window movement to cordinates</p>
<p>receiving: nothing happens</p>

Let me know whether something is wrong.