Implement pp6calculator Exercise 1 solution

Add main program for pp6calculator, and perform the classic "Hello
World" output to complete Exercise 1.

Update README with system requirements and build instructions using
the GNU/Clang compilers.
Implement pp6calculator Exercise 2 solution

Query user to enter two numbers, and report product of numbers to
stdout.
Implement pp6calculator Exercise 3 solution

Provide interface to allow user to select one of the four arithmetic
operations and then input two integers. Apply the operation to the
numbers and report the result to stdout.

Check input for errors at all stages, and report these to stderr before
exiting with a failure.
Implement pp6calculator Exercise 4 solution

Wrap selection of arithmetic operator, numbers and result output in
while loop to allow repeat operations. Exit loop on user entering
'q' as operator.
Implement pp6calculator Exercise 5 solution

Refactor arithmetic operations into functions taking the input
arguments and returning the result. Calculate results using these
functions.