Add GPL v3 license.
Minor correction.
Add makefiles.

Trying different makefiles lead to the conclusion that the simplest
structure is to have a separate folder for every exercise. Currently the
`Makefile.Exercise` provides a reusable makefile for every exercise. In
time, if it will become required, the solution will evolve, possibly to
makefiles in the exercise folders.
Add exercise from 1-1 through 1-16.