Controller Gas Proxy and CHI token integration

[nostdm]

Changes

• Created a gasProxy.sol contract which allows the controller to execute a transaction and have some of it's gas refunded using gas tokens.
• Created a corresponding gasRefundable.sol contract which integrates with the CHI gas token by default.
• Opted to use the gas token address directly instead of resolving ENS to reduce gas cost overhead.
• Gas token address and parameters are settable by the controller in case where we need to change the gas token to a different one.
• Added gasToken.sol and gasBurner.sol mock contracts to help with testing.
• Added the new contracts to the various CI jobs.
• Formatted the new contracts with prettier.
• Added tests cases for the gas proxy and gas refundable settings.

Testing

• Ran the security tools on contracts related to the gas proxy.
• Successfully ran existing gas proxy and gas refundable test cases.
• Tested the gas token and gas proxy integration on Ropsten and the Development network.

Update

• Opted to use structs in method parameters and return values to make the interface cleaner.
  Note that this requires the use of pragma experimental ABIEncoderV2; even though since 0.6.0 it's no longer experimental:

You still have to explicitly activate it using pragma experimental ABIEncoderV2; - we kept the same pragma, even though it is not considered experimental anymore.

[mischat]

I think there are some changes in there, with my comments, that we should discuss or change accordingly.