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
`(if condition then then_fn else else_fn) cb` is awesome.

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