Add program that implements counting sort

[faheel]

Name the source file RadixSort.cpp and add it to the Sorting directory.

You should use the utility functions from SortingUtils.h.

For an overview of how the code should be structured, view implementations of other sorting algorithms, and also go through the C++ Coding Guidelines, if your implementation is in C++.

Also add proper comments!

References:

1. Counting sort - Wikipedia
2. Counting Sort - GeeksforGeeks

[faheel]

Closed in #6.