BuyTokens getting rate from a function instead of field. Useful for making Crowdsales with bonus periods

[Perseverance]

I've changed the buyTokens method to get the rate from getRate function. This function can later be overridden to make crowdsales with different rate periods based on some business logic.

[Perseverance]

Sorry for the messiness with the commits, we should squash them upon merge

[shrugs]

LGTM, but since this is a breaking change we should settle on a semver strategy before merging (and it should be rebased on to master; 1.5.0 now).

cc @frangio for the semver strategy.

[Perseverance]

Correct me if I am wrong, but I rebased already and saw the version is 1.4.0 before upgrading. Am I rebasing to the wrong branch?

[shrugs]

You're totally right; we forgot to update package.json when we released 1.5.0 a few days ago. https://github.com/OpenZeppelin/zeppelin-solidity/releases

[grimsa]

Wouldn't extracting getTokenAmount(weiAmount) with default implementation of weiAmount.mul(rate) be more flexible?

[Perseverance]

I am not really sure which one is more flexible :) I wanted to keep the code as similar to the original one

[grimsa]

I had two scenarios in my mind:

1. Applying multiple price levels to a transaction, for example:
   • You have a crowdsale with a bonus period where you are minting tokens at 25% increased rate until GOAL wei are raised.
   • Let's say you have raised (GOAL - 10 wei) already and a big transaction comes sending 10k ETH. You only want to mint (10 wei * 25%) worth of bonus tokens (not 10K ETH * 25 %).
2. Rounding problems when original rate = 1. Example:
   • If you want to implement 1 ETH = 1 Token crowdsale, you'll use a rate of 1 and a token with 18 decimal places.
   • Now if you want to issue tokens at 125% rate at first, you have a problem, as getRate result will be 1 (original rate) * 5/4, which rounds down to 1, hence, no bonus.

It is impossible to implement any of these two by just overriding getRate().

As you can always override getTokenAmount(weiAmount) and implement it as weiAmount.mul(getRate()) with a custom getRate() function, to me it seems like a superior approach.

[Perseverance]

You are right, will rewrite it ASAP ;)

[Perseverance]

@grimsa @shrugs Moved this pull request here for clarity purposes:
#638