Add gaslimit upper bound of `2**63-1` EIP 106
Add gaslimit upper bound of `2**63-1` EIP 106

For reference:
- https://github.com/ethereum/EIPs/issues/92
- https://github.com/ethereum/EIPs/issues/106

karlfloersch/pyethereum,shahankhatch/pyethereum,karlfloersch/pyethereum,ethereum/pyethereum,shahankhatch/pyethereum,ethereum/pyethereum