Experimental GPU miner for BGL

This is small, but working GPU miner from community, based on pretty
old CPU miner with GPU capabilities added. The useful aspect of
it is its small code size, that does not contain multi-coin logic
and other features of modern larger miner software. This means
it can be easily tweaked and modified, good for studying various concepts.
Currently is built for Win, but with small effort a linux port
can be created too.

Sources and Win64 binaries are provided.