Adds a few methods for setting additional data

[imartingraham]

Currently it's not possible to set the payment action, the tax amount, max amount or the shipping amount without just setting setting the payOptions object explicitly like this:

paypal.payOptions = {
    PAYMENTREQUEST_0_SHIPPINGAMT: 4.99,
    PAYMENTREQUEST_0_PAYMENTACTION: 'Authorization',
    PAYMENTREQUEST_0_TAXAMT: 12.45,
    MAXAMT: 99.00
}

This pull request adds methods for adding those using methods to avoid accidentally overwriting any of the other options set on the payOptions object.

paypal.setShippingAmount(4.99)
    .setPaymentAction('Authorization')
    .setTaxAmount(12.45)
    .setMaxAmount(99.99);