Added new configuration option STRATUM_MINING_PROCESS_NAME.

If you run multiple instances of stratum mining it's nice to set a process identifier / name. 

With this small change you can easily find the right process via ps on console.

```shell
stratum     11306  0.2  1.1 387428 23848 ?        Sl   10:17   0:03 twistd-stratum-mining /usr/local/bin/twistd --originalname -y launcher.tac
```