Capture amounts as strings so raw value available to client apps

Adds AmountStr and ServiceFeeAmountStr fields to Transaction as strings to capture the original data sent by Braintree. Populate the existing Amount and ServiceFeeAmount fields by parsing the string fields for backwards compatibility.

Clients can avoid rounding errors by parsing the string versions of the fields themselves.
No, AmountStr is read as a string from the original XML so it will be "4.1", then Amount is parsed from that.