Create our own signedExtension to handle transaction fees

Due to #77 (closed) , we can't reuse pallet_transaction_payment::ChargeTransactionPayment. OS, we must create our own SignedExtension.

I think the easiest way is to fork pallet pallet_transaction_payment and keep the same code except for priority, just set the priority to zore and let another SignedExtension handle the priority.

We need to fork also the crates that defines custom rpc and runtime api, and use the custom rpc from our fork, so this also impact the binary.

Another solution: convince parity to add a trait that allow to customize how to compute priority.