Improved transaction pool

The transaction pool will now some easily be able to pre determine the
validity of a transaction by checking the following:

* Account existst
* gas limit higher than the instrinsic gas
* enough funds to pay upfront costs
* nonce check

miner/worker.go

@@ -288,7 +288,7 @@ gasLimit:
 			tcount++
 		}
 	}
-	self.eth.TxPool().InvalidateSet(remove)
+	//self.eth.TxPool().InvalidateSet(remove)
 
 	var (
 		uncles    []*types.Header