Added receipt_email to Charge + Fixed Examples Formatting

[elithrar]

Added the new receipt_email field to the Charge object.

It allows the API user to specify an email address to send a receipt to in place of the Customer's stored address, or in cases where no Customer has been specified.

I've also fixed some minor formatting issues with the examples (which somehow got pulled into this branch—I can pull them into a separate PR if required).