Pass request object to #build_mail

This makes it easy to access the request object (and therefore the URL params, headers, etc) in actions by overloading the default #build_mail and handling the second (request) parameter.

This is related to #50, but is more lightweight and puts access to the params at the business logic layer (where they more properly belong) instead of the template layer.

This commit does not change the API or impact existing functionality.