Timing events

This patch measures the time between each important part of the loop:

* The length of each "EnterFrame" and "RenderScene" event
* The time Crafty waits from the end of one to the beginning of the next

The time since the last frame is passed to the "EnterFrame" event as the property dt.

I also moved the docs for "EnterFrame" to Crafty.timer, since that's where the events happen.