Improve merge sort implementation

[lamnguyen2187]

The current implementation has space complexity of O(NLogN) because it creates temp storage at each recursion level. This can be avoided by creating a temporary working area with a size as large as the array to be sorted and then passing it down to each recursion level. The complexity will be reduced to O(N).

[coveralls]

Coverage increased (+0.01%) to 96.602% when pulling 3274b4b on lamnguyen2187:mergesort into 2084c96 on felipernb:master.