Add tests for array pair sum and a working solution.

[stezu]

The previous solution didn’t allow for duplicate results so it didn’t
work as expected by the second test. This solution works with both
tests and does so very quickly at ~1.4M ops/sec
(http://jsperf.com/array-pair-sums).

[blakeembrey]

Hi @stezu, I actually think the readme example is wrong since I remember spending a bunch of time perfecting not outputting duplicate elements 😛

Source: https://github.com/blakeembrey/code-problems/tree/master/problems/array-pair-sum

[stezu]

The expected result of the second test is going to be [ [3, 5], [4, 4] ] then right?. In that case, neither of our solutions work. :P

[blakeembrey]

It should be just [[3, 5]]. But we should update the example to match the description.

[blakeembrey]

I've merged but I'm going to refactor as your solution is also O(n^2) regardless of being right/wrong according to the README.

Edit: Sorry, zoned out then. It's O(log * n).