Change gas logic (Qtum Core / QTUMCORE-72)

[designsters]

We need a minor change into how we handle gasLimit and gasPrice in our current implementation in light of issue #113
The changes must be made on Qtum side, no change to the EVM submodule should be required

The new logic will be as follows:
1- when a Qtum tx is received, we make sure the fee covers gasLimit*gasPrice otherwise reject it
2- we pass the gasLimit to the EVM and pass 1 as gasPrice
3- after we get the execution results, two cases:

if an exception occurs (including out of gas), we consume all the gas (gasLimitgasPrice)
if the transaction is successfully executed, we consume gasUsedgasPrice and send refund (gasLimit-gasUsed)*gasPrice ...

This will allow us to use gasLimit as intended in Qtum, and use gasPrice after the execution to charge the right values to the sender.

Example:
for example, if we call a contract with a gas limit of 50000 and a price of 0.00001, first we check if the tx fee is >= 500000.00001=0.5 then we pass 50000 to the EVM as gasLimit and 1 as gas price, let’s say the gas used was just 20000, then after execution, we should pay the miner 200000.00001=0.2 and refund the sender 30000*0.00001=0.3