Add if flow

Adds something similar to what is discussed here http://joseoncode.com/2012/06/24/messing-with-cps-in-js/ and what we talked about https://github.com/Clever/clever-common/pull/68

Allows us to write that code to https://github.com/Clever/clever-common/pull/69
since the convention is for the first arg to callbacks to be an error, it might be more useful to demonstrate the behavior with an error and/or result(s)