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/mist/ui_lib.go

@@ -159,7 +159,7 @@ func (self *UiLib) RemoveLocalTransaction(id int) {
 
 func (self *UiLib) ToggleMining() bool {
 	if !self.eth.IsMining() {
-		err := self.eth.StartMining()
+		err := self.eth.StartMining(4)
 		return err == nil
 	} else {
 		self.eth.StopMining()