Expands the *Amount values in the Pay/Refund/Reserve/Settle/SettleDebt/WriteOffDebt calls.

TransactionAmount, RefundAmount and SettlementAmount where not being expanded out to include the required Amount fields. Expanded them out to include Value and CurrencyCode sub fields required by Amount fields.
MarketplaceFixedFee is also a complex type.
Merge pull request #5 from colinmathews/master

Expanding on mphilpot's complex type to include MarketplaceFixedFee