Update the bender API to look more like jbender

[cgordon]

The API for bender was created before the API for jbender (see https://github.com/pinterest/jbender). The API for jbender is more elegant and requires a LOT less memory allocations in the main loop (as it sends far fewer events). This would be a non-backwards compatible change, but given the way people vendor dependencies that should be alright for existing users.

[nuance]

The API should probably also be tweaked to use time.Duration and other non-numeric types where appropriate.

[cgordon]

Agreed, in fact, I made very poor use of the Go type system in all of this project, and it is due for a major overhaul.