accept only multiplies of token price as commitment amount

[rudolfix]

currently any amount can be accepted by ETO
number of tokens per amount is always rounded down to prohibit "shaving" single tokens by investing `amount + token price/2 +1 wei) which usually we round up
in low gas times it could be significant attack

this however will take any rounding surplus from investor and give to company. amounts are not significant but to remain 100% fair we should only accept amounts "divisible" by token price (up to a small limit). large differences should revert