separate miner

Mining, force mining, mining new transactions, etc. function properly along with start/stop operations. 
While functions and lambdas are for sure a cool bit of C++11, this is quite a flimsy mechanism for what is really a fundamental portion of the architecture of the Miner/Client class interaction.

Better to separate it out to a method of the Client class (noteMiningDone) and have it call that on the callback.