core: added basic chain configuration

Added chain configuration options and write out during genesis database
insertion. If no "config" was found, nothing is written to the database.

Configurations are written on a per genesis base. This means
that any chain (which is identified by it's genesis hash) can have their
own chain settings.

cmd/geth/usage.go

@@ -121,6 +121,7 @@ var AppHelpFlagGroups = []flagGroup{
 		Flags: []cli.Flag{
 			utils.MiningEnabledFlag,
 			utils.MinerThreadsFlag,
+			utils.TargetGasLimitFlag,
 			utils.MiningGPUFlag,
 			utils.AutoDAGFlag,
 			utils.EtherbaseFlag,