Add an explanation of the peano axioms.

I still need to cite the source (Landeau's book).
Unchange the margins

This actually makes the book much more readable, and printer-friendly.
Style changes to peano axioms chapter

* Fixed bad references to axioms
* There was a bolded word, which wasn't prominent enough, so I italicized it
  instead.
\nocite Landau's book
\nocite LYAH.
Build script now terminates if first latex compile fails.
Merge branch 'f/peano' of github.com:pharpend/lysa into f/peano
Outline sets chapter

The peano axioms fit in nicely with the sets chapter, so I moved them there. I
also outlined what else needs to be written in the sets chapter.
stubbing out contrib guide
fixing formatting
removed gitflow branching discussion
Merge branch 'develop' into f/peano
Merge branch 'f/readme-travis' into f/peano
Merge branch 'f/readme-travis' into f/peano
Merge branch 'develop' into f/peano
Merge branch 'f/readme-travis' into f/peano
Merge branch 'f/peano' of ssh://github.com/pharpend/lysa into f/peano
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.