ADDED: Merge Sort, Testing

Done Merge Sort in Go. Added function Top to the stack to be used
by Merge sort.
Initially I planned on using plain lists like Cormen's pseudo-code
but found doing the merge part with stacks makes a lot of stuff simpler
and it's much more intuitive.
Added basic testing and modified insertion sort for testing
Still hoping to making sorting algorithms more generic but that
wouldn't make a lot of sense in the practical sense because Go's
sort package does a great job. If you're not looking at these for
studying purposes, go with the sort package that uses an exquisite
combination of insertion sort, quicksort and heapsort, depending on
the case, giving you constant O(nlogn).