Merge the develop branch to the master branch, preparation to v2.3.0

[akolotov]

This merge contains the following set of improvements and fixes:

• Enhancement: ERC677 bridgeable token can be used by BlockReward contract (Extend ERC677BridgeToken contract for xDai-POA bridge #129, Override transfer and transferFrom in ERC677BridgeTokenRewardable #153, Rename ValidatorSet contract to Staking contract #157)
• Enhancement: Add ERC677 support on foreign bridge in erc20-to-erc20 bridge mode (Add ERC677 support on foreign bridge in erc20-to-erc20 bridge mode #177)
• Enhancement: Dynamic gas limit for the deployment/configuration transactions (Dynamic gas limit for the deployment/configuration transactions #178)
• Enhancement: Reward for the bridge validators (Update contracts to properly traverse validators reward accounts tests #130, Add Fee Manager implementation for ERC20-to-Native mode #132, Update deploy script to include Fee Manager options for ERC-TO-NATIVE mode #134, Update ERC-TO-NATIVE gas consumption docs #135, Add reward management schemes #141, Add Fee Manager implementation for Native-to-ERC mode #142, Update gas consumption NATIVE-TO-ERC #144, Add fee manager mode interface #150, Separate fee into home fee and foreign fee #151, Add fee manager for Native-to-ERC that collects fees on home network  #155, Add Fee Manager for erc-to-native bridge in POSDAO chain #164, Add fee manager for erc-to-erc bridge in POSDAO chain #166, Add validatorList on BaseBridgeValidators #167, Update RewardableValidators to be compatible with BridgeValidators  #169, Emit event on fee distribution from transfer #171, Fee distribution events in the RewardableBridge contract #176, flattening sources of the new set of contracts #179)
• Enhancement: Native-to-ERC20 bridge with the Ethereum Classic as Home chain (Support transaction null status on deploy script #156, Flattening of contracts related to the bridge on Ethereum Classic  #184)
• Enhancement: Support multiple evm versions (Migrate contracts to solidity version 0.4.19 #161, Support multiple evm versions #181)
• Fix: Replace default web3 provider URL in .env.example (Replace poa.infura.io with core.poa.network #152)
• Fix: Upgrade Node.js version to 10 (Upgrade Node.js version #154)
• Fix: Avoid calling blockReward if zero value on foreign transfer on erc-to-native (Avoid calling blockReward if zero value on foreign transfer on erc-to-native #165)
• Fix: Updating README with clarifications for usage FOREIGN_DAILY_LIMIT and FOREIGN_MAX_AMOUNT_PER_TX (update FOREIGN_DAILY_LIMIT & FOREIGN_MAX_AMOUNT_PER_TX for bridge modes #172)