Add support for perfect audience ecommerce params

Would potentially close #406 

But this needs careful review and attention, seeing as its my first, and most likely does not have sufficient tests. (edit: they failed ;p)
 
Would love to go over my thinking in person with one/more of you tomorrow. @ianstormtaylor @reinpk @calvinfo @lancejpollard 
Awesome, sounds good to me! Async API's are the way to go.