Core limiting feature added to miner

Recently, miners have requested the ability to specify the number of cores used for mining. This release includes that feature.

To use:
Ubuntu
python p2p.py config.json --cores [number-of-cores]
Example:
python p2p.py config.json --cores 8

Windows
yadacoin.exe config.json --cores [number-of-cores]
Example:
yadacoin.exe config.json --cores 8