Use Etherscan API for contract code verification

[vinceau]

Description

This PR introduces the yarn run verify command for verifying deployed contracts.

Motivation

This allows for automated contract verification without having to interact with the Etherscan frontend for verifying contracts.

Design

The yarn run verify command requires the name of the contract to be passed in as a parameter. This makes it hard to additionally pass in the network parameter too. Previous we had two deploy commands, one for Kovan and one for Mainnet. In replacement of that we introduce the NETWORK environmental variable, that should be passed in with the command.

For example:

NETWORK=kovan yarn run deploy

or

NETWORK=mainnet yarn run verify DarknodePayment

Unresolved Issues

Ideally, the contract verification should not be a separate step but should be a part of the deployment stage. After each contract deploy, that contract should automatically be verified, rather than having to specify which contracts to verify. This can be done by extending the truffle-deployer and adding a post deploy hook of some kind, in combination with sol-merger or truffle-flattener and the Etherscan API interface in this PR.