URI QR ABI ETH functionality

This is no longer 2017, add support for Ethereum contracts with existing ETH wallets by allowing EIP-67 compliant URIs for payment requests (e.g. via scanned QR code) with a data payload that includes the ABI encoded function name and encoded function arguments.

Start by reading the EIP-67 URI requirements and reviewing how MetaMask currently handles payment requests and scanned QR codes. Define the supported URI and ABI payload behavior, then verify that existing ETH wallets can process contract payment requests with encoded function names and arguments.

Written by the indexing model from the issue text.