Modified to count the pairs that sum to zero (twoSum).
Updated comments to describe the Two Sum quadratic algorithm.
Implemented a faster algorithm for finding pairs of integers that sum to
zero (twoSum). The algorithm first sorts the input array in place. Then,
for each integer in the array, it proceeds to find the array index of an
integer which might cancels out the subject integer (i.e. sum to zero). 

If an integer is found which cancels the subject integer, AND it appears
after the subject in the sorted array, count is incremented.
If, however, the cancelling integer appears before the subject integer,
it's not counted, to avoid double counting.
Added comment to describe the NlogN sorted-binary search algorithm.
Restored ThreeSum and TwoSum from local history.
Commented that the assumption for this algorithm is that all integers
are unique.