support per worker difficulty settings

[toddfries]

I have three antminer s3's and only one can overclock to achieve the blanket difficulty of 512GH/s .. the other two would be better served at the 256GH/s difficulty level .. I'm sure there are other similar scenarios; I could mine to a different address and have a per address difficulty level setting as a start, if eligius can do such, ...

This is clearly a feature request and not a bug report.... ;-)

[toddfries]

given that I now know that the work size is auto configured based on observed performance, I think this is now more than uinteresting.