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
some more concise/clear options: 

```coffee
if condition then then_fn cb else else_fn cb

(if condition then then_fn else else_fn) cb

if condition
  then_fn cb
else
  else_fn cb
```