add sometimes feature including tests

[richriscunha]

#69

[iamdtang]

on vacation. will check this in a week.

[garygreen]

@richriscunha thanks for this PR. Just one thing, could you also add the:

if (this._passesOptionalCheck(attributeRules) && !(this._suppliedWithData(attribute))) { continue; }

to the checkAsync method as well? Bit funny that there are two check methods but one is to handle async-based validation.

[richriscunha]

Sure, I'm traveling on business now. So, I won't get to it until at least the middle to end of the week. But, if I can get to it sooner, I will.

[richriscunha]

@garygreen I've added the additional check to the checkAsync method now too. In addition, I've added a couple of more test specs to cover checkAsync using the sometimes rule too.