Added ajax throttling when tab is inactive.

The current implementation might not be that interesting, but maybe it could come in handy some day. :)

* Added a window blur/focus listener to main.js, that sets the data property "isInactive" on the jquery window object to true/false. This property should checked using === as its initial state is undefined!
* Added a check if current window is active to the setTimeout of the updateTimes function.