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 library 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 redundant 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)