Extend PaymentSplitter to support ERC20 tokens

[Amxx]

This PR extends the existing PaymentSplitter to add support for ERC20 tokens.

New functions and events include:

Ether	ERC20
function totalReleased() public view returns (uint256)	function totalReleased(IERC20 asset) public view returns (uint256)
function released(address account) public view returns (uint256)	function released(IERC20 asset, address account) public view returns (uint256)
function release(address payable account) public	function release(IERC20 asset, address payable account) public
event PaymentReceived(address from, uint256 amount)	event ERC20PaymentReleased(IERC20 indexed asset, address to, uint256 amount)

Fixes #2701, #2762, #2853

• Note: storage layout is maintained, upgradeable instances of PaymentSplitter can safely be updated to use this new feature

PR Checklist

• Tests
• Changelog entry

[frangio]

The @dev documentation at the contract level needs to be updated. Please include a mention that rebasing tokens may not be distributed equally. And I'm wondering if fee on transfer tokens would even work, if they don't we should document that as well.

[AboldUSER]

Glad you all are updating this to paymentsplitter for ERC20s. I did this for AirSwap last month, and released a tutorial (with its own github repo) for others to create their own tokenpaymentsplitter contracts.
https://medium.com/coinmonks/create-an-erc20-token-payment-splitting-smart-contract-c79436470ccc

Will be great when you integrate the ERC20 functionality into the main repo.

[sabotagebeats]

great to have this included in the main repo!