Easier post_send API

[jtpereyda]

The post_send API is defined by a function, and it's not exceedingly intuitive. Consider using an interface instead.

Inspired by this talk: The End Of Object Inheritance & The Beginning Of A New Modularity

Essentially, Python is not defined to handle functions this way. It can, but the language lends itself to using objects rather than just functions. One of the speakers argues that objects/interfaces are better than functions for this case.

The current way of registering post_send is also pretty clunky. We could take a more event handler style approach and allow multiple functions.

I'm not sure what the best approach is. This should be settled after we see more use cases.