Add more customizable options for the BTDropInViewController in showPaymentViewController()

[whizkid77]

It would be nice to support the following:

title, summaryTitle, summaryDescription, displayAmount, and callToActionText

to allow for more customization of the drop-in view controller, particularly for showPaymentViewController().

Example js usage:

BTClient.showPaymentViewController(title,summary_title,summary_description,display_amount,call_to_action_text,callback);

This would be a breaking change to the function signature though.