New OpenLayer "data decay" functionality. Great for real time streams.

The OpenLayers heatmap layer is great for displaying real time data points coming in across a stream.  However, when the real time stream is very very busy, the data store backing the heatmap can get too large for the browser to handle, or just too chatty for the data to be meaningful to the end user.

I've added code to easily limit the data being displayed in two ways. 
1. A maximum number of data points allowed to be shown.
2. A maximum time window.  

By setting a 'maxSize', or 'maxTime', the oldest data is removed (decayed) off of the heatmap.

I also built an example demo for each usage.  Hopefully this can be incorporated somehow into the master branch, and is useful to other projects needing to limit the data being displayed on a heatmap. Enjoy!