Emit Approval event on allowance change

[k06a]

More details here: makerdao/dss#76

[MicahZoltu]

I mildly disagree with the reasoning provided in the linked discussions. I do think there is value in emitting an event for transferFrom that is unique from transfer. I don't agree that approval is the right such event. ERC20 is terrible in many ways, but I don't think the solution to that is for people to silently extend ERC20 without any attempt at creating an actual better standard (like a set of events that provides all of the information one may want, rather than transfer/approval events only). I have lobbied in the past for people using ERC777, but if that is truly hated (which I believe it is by the Uniswap team) then some other standard would be better to follow rather than ad-hoc compatibility with a random subset of tokens in the space.

That being said, since it seems the dapp space is split on this topic, you are forced to pick a side and following OZ/Maker isn't a bad choice.

[k06a]

@MicahZoltu I agree, any new ERC20 implementors should pick a side or at least be notified of existing differences in implementations.