Changelog

2.2.0 (unreleased)

ERC20: added internal _approve(address owner, address spender, uint256 value), allowing derived contracts to set the allowance of arbitrary accounts.
ERC20Metadata: added internal _setTokenURI(string memory tokenURI).
ERC20Snapshot: create snapshots on demand of the token balances and total supply, to later retrieve and e.g. calculate dividends at a past time.

Upgraded the minimum compiler version to v0.5.2: this removes many Solidity warnings that were false positives.
Counter's API has been improved, and is now used by ERC721 (though it is still in drafts).
ERC721's transfers are now more gas efficient due to removal of unnecessary SafeMath calls.
Fixed variable shadowing issues.

Removed most of the test suite from the npm package, except PublicRole.behavior.js, which may be useful to users testing their own Roles.

Now targeting the 0.5.x line of Solidity compilers. For 0.4.24 support, use version 2.0 of OpenZeppelin.
WhitelistCrowdsale: a crowdsale where only whitelisted accounts (WhitelistedRole) can purchase tokens. Adding or removing accounts from the whitelist is done by whitelist admins (WhitelistAdminRole). Similar to the pre-2.0 WhitelistedCrowdsale. (#1525, #1589)
RefundablePostDeliveryCrowdsale: replacement for RefundableCrowdsale (deprecated, see below) where tokens are only granted once the crowdsale ends (if it meets its goal). (#1543)
PausableCrowdsale: allows for pausers (PauserRole) to pause token purchases. Other crowdsale operations (e.g. withdrawals and refunds, if applicable) are not affected. (#832)
ERC20: transferFrom and _burnFrom now emit Approval events, to represent the token's state comprehensively through events. (#1524)
ERC721: added _burn(uint256 tokenId), replacing the similar deprecated function (see below). (#1550)
ERC721: added _tokensOfOwner(address owner), allowing to internally retrieve the array of an account's owned tokens. (#1522)
Crowdsales: all constructors are now public, meaning it is not necessary to extend these contracts in order to deploy them. The exception is FinalizableCrowdsale, since it is meaningless unless extended. (#1564)
SignedSafeMath: added overflow-safe operations for signed integers (int256). (#1559, #1588)

The compiler version required by Array was behind the rest of the libray so it was updated to v0.4.24. (#1553)
Now conforming to a 4-space indentation code style. (1508)
ERC20: more gas efficient due to removed redundant requires. (#1409)
ERC721: fixed a bug that prevented internal data structures from being properly cleaned, missing potential gas refunds. (#1539 and #1549)
ERC721: general gas savings on transferFrom, _mint and _burn, due to redudant requires and SSTOREs. (#1549)

ERC721._burn(address owner, uint256 tokenId): due to the owner parameter being unnecessary. (#1550)
RefundableCrowdsale: due to trading abuse potential on crowdsales that miss their goal. (#1543)