* rename {increase,decrease}Approval to {increase,decrease}Allowance
* add non-zero spender check to approve and {increase,decrease}Allowance
* Updated tests to reflect the new behavior.
* fix wrong test description
* fix old function names
* Fixed linter error.
* Fixed typo.
* rename ERC20 to IERC20
* move ERC20.sol to IERC20.sol
* rename StandardToken to ERC20
* rename StandardTokenMock to ERC20Mock
* move StandardToken.sol to ERC20.sol, likewise test and mock files
* rename MintableToken to ERC20Mintable
* move MintableToken.sol to ERC20Mintable.sol, likewise test and mock files
* rename BurnableToken to ERC20Burnable
* move BurnableToken.sol to ERC20Burnable.sol, likewise for related files
* rename CappedToken to ERC20Capped
* move CappedToken.sol to ERC20Capped.sol, likewise for related files
* rename PausableToken to ERC20Pausable
* move PausableToken.sol to ERC20Pausable.sol, likewise for related files
* rename DetailedERC20 to ERC20Detailed
* move DetailedERC20.sol to ERC20Detailed.sol, likewise for related files
* rename ERC721 to IERC721, and likewise for other related interfaces
* move ERC721.sol to IERC721.sol, likewise for other 721 interfaces
* rename ERC721Token to ERC721
* move ERC721Token.sol to ERC721.sol, likewise for related files
* rename ERC721BasicToken to ERC721Basic
* move ERC721BasicToken.sol to ERC721Basic.sol, likewise for related files
* rename ERC721PausableToken to ERC721Pausable
* move ERC721PausableToken.sol to ERC721Pausable.sol
* rename ERC165 to IERC165
* move ERC165.sol to IERC165.sol
* amend comment that ERC20 is based on FirstBlood
* fix comments mentioning IERC721Receiver
