cmd/geth, cmd/utils, eth: added dev mode flag

Dev mode enabled some debugging flags such as:

* VM debugging mode
* Simpler proof of work
* Whisper enabled by default
* Datadir to a tmp datadir
* Maxpeers set to 0
* Gas price of 0
* Random listen port

cmd/geth/main.go

@@ -308,6 +308,7 @@ JavaScript API. See https://github.com/ethereum/go-ethereum/wiki/Javascipt-Conso
 		utils.IPCPathFlag,
 		utils.ExecFlag,
 		utils.WhisperEnabledFlag,
+		utils.DevModeFlag,
 		utils.VMDebugFlag,
 		utils.VMForceJitFlag,
 		utils.VMJitCacheFlag,