Refactor exercise

[thgaskell]

I think that this exercise deserves a makeover. I think the mix of rounding errors, number parsing, and wordy descriptions take away from the intended nature of the exercise, which is using array iterator methods.

The exported json value also gets cached, and if it's modified via reassignment, it carries over to the next tests which produces some weird side effects when running with the mocha watch task.

In addition, testing the values of global variables is kind of clunky because the tests are just testing for a certain value, rather than if the code is doing the correct things. Exported functions could allow for variable cutoff points for filtering, tax values, and states to filter by.