Wallet: determine valid amounts from mint's keys response

[callebtc]

Currently, the wallet assumes that the mint will use amounts in 2^n. This is not necessarily true, as the mint could use any amount decomposition they like (which is fine according to the protocol).

The wallet should not make use of settings.max_order but infer this from the keys it has received from the mint. An example line in wallet.py that needs to be fixed is:

allowed_amounts = [2**i for i in range(settings.max_order)]