cmd/geth, miner, backend, xeth: Fixed miner threads to be settable

Miner threads are now settable through the admin interface (closes #897)
and specify 0 CPU worker threads when eth_getWork is called (closes #916)

cmd/geth/main.go

@@ -401,7 +401,7 @@ func startEth(ctx *cli.Context, eth *eth.Ethereum) {
 		}
 	}
 	if ctx.GlobalBool(utils.MiningEnabledFlag.Name) {
-		if err := eth.StartMining(); err != nil {
+		if err := eth.StartMining(ctx.GlobalInt(utils.MinerThreadsFlag.Name)); err != nil {
 			utils.Fatalf("%v", err)
 		}
 	}