Increase usefulness of sharelog by reporting difficulty levels of shares

Add some more information to the sharelog script:

4997200,accept,ffffffffffffffffffffffffffffffffffffffffffffff4b3789410000000000,stratum+tcp://54.206.71.186:9332,PXY16,16,5f844a9c8d58a82f9c050993944a85ebe9d5d02469fdd490f04f2b0000000000,000000027ced44666ce97d508cc41ff9a824192de76a0af72142c9dc0000000000000000aca88457ed4606de7be505675a95e0d9f3f9c2b578333a110db387852a3254925387942018692842dcbd2626000000800000000000000000000000000000000000000000000000000000000000000000000000000000000080020000,1.51k/1k,1513.104908,1000.000000

Note the additional human-readable difficulty, plus the floats of the generated and requested difficulty. This is to ease post-processing of sharelogs later.