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
Setup Travis CI build system.
Add in hopefully correct dependencies.
Add back xetex.
Build script now terminates on error.
Merge branch 'f/terminate-build-script' into upstream/build-system
The "script" field no longer does cd nonsense.
Add texlive-math-extra as a dependency
Add the cd stuff back in.

it turns out it wasn't pointless. it was needed because the build script assumes
you are in its directory.
Add texlive-latex3 as a dep