Port the lessons to SFML2 + CMake

Also many small changes, such as
- Cleaning-up the use of Title Case in files, variables, and functions
- Lessons are now named such that they will lexically sort correctly
- The 'App' variable is now 'myWindow'

There's more to do (see TODO.txt), but all the 'lesson' files should
compile and run in Linux and in OS X. The building.txt file should help.
(Windows is not yet tested, and the non-lesson cpp files aren't yet ported.)