Set up buffered pruning of stops to allow a window beyond the sample period

This change fixes the bizarre 'real-time' values that were previously seen

![screen shot 2014-12-04 at 12 44 55 pm](https://cloud.githubusercontent.com/assets/1977405/5302988/6431ad2a-7bb3-11e4-8eaa-3fcaa3bf646e.png)