Add convenience support for  Pay

[danthorpe]

PKPaymentRequest has properties for a currency code and array of PKPaymentSummaryItem which in turn composes an NSDecimalNumber.

Therefore we can provide an equivalent value type for a MoneyType, label, and item type. With a convenience initializer on PKPaymentRequest.

[codecov-io]

Current coverage is 100.00%

Merging #23 into development will not affect coverage as of 6a40aa1

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+ Hit                 572     643    +71
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Review entire Coverage Diff as of 6a40aa1

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