Set key event timestamp

This is a fix for the issue mentioned here: https://github.com/libgdx/libgdx/issues/1576#issuecomment-38982627

Furthermore, the request to add a Input#clearEvents() method is quite recurrent and the answer is always a won't fix  (for good reasons though).
So do you think that the InputConsumer proposed at the link above might deserve its place in gdx ?
If so I'll make a PR soon.