A set of amount preference fixes

[callebtc]

Some issues I think are worth fixing:

• Allow setting the preference for all outputs of a swap (createSplitPayload), not only for proofsToSend (but also proofsToKeep)
• splitAmount seems to "fill up the rest" with 2^n even if amountPreference is set. Is this intended?
• Do not check if all amounts are 2^n, since this is not generally the case. Replace this check by checking if every amount has a key.
• Consider replacing the AmountPrefernce object { amount: int, count: int } with a simple number[] that holds the requested amounts (feels cleaner to me).

[lollerfirst]

My new PR addresses first 3 points. I take it that point 4 is to be discussed.