Merge pull request #18 from pharpend/f/readme-travis

Add travis icon to the readme
Merge branch 'develop' of github.com:learnmath/lysa into f/peano
Add some stuff about intuitive sets.

It's not finished yet, but I'm committing for the sake of, well, I'm not sure.
But I'm committing.
Add placeholder for conventions section.

I have a feeling we will need it. It will be just as useful for the reader as it
will be for the contributor. Sorry for my bad grammar, I'm about to fall asleep.
Add placeholder files for chapter 3.

If you compile the book, you'll see there's already a chapter 3. That's because
I decided to devote all of chapter 2 to sets, we'll get to the special types of
sets in chapter 3.

Anyway, I was too lazy to move the chapter 3 text to a new file, so, for the
time being, I just `touch`ed the files, and will use them as placeholders.

I'll move the text tomorrow morning.

Night, everyone.
Exercises and resp. solutions each have own page.

Each set of exercises starts on a fresh page now, as are the solutions. This was
already the case with the solutions. Furthermore, the text following the
solutions starts on a fresh page.