Added error amplitude filtering for DigiShield difficulty control.

This change adds error amplitude filtering with 8-factor reduction in the proposed Digishield difficulty controller.   All error metrics are lower than original Digishield implementation, including multipool mining time.  This appears to keep the difficulty just slightly above the MP profit threshold.
![digishield-filter8](https://f.cloud.github.com/assets/211715/2396067/43e37d08-a9d3-11e3-8ad0-aecfa85d35fe.png)

Original implementation for reference.
![digishield-nofilter](https://f.cloud.github.com/assets/211715/2396070/5ccc98ea-a9d3-11e3-9543-38d78c9211c9.png)