Ignore latexmk files; updated Makefile
Organized into sections; added some math

Most of the work was to refactor some parts of the introduction into
more sections, giving it a more "organized feeling". Removed some text
as well, for it was more boilerplate than necessary.

Added some equations to the section talking about Markov processes and a
definition of trigram language models to the NLP chapter. Also added a
new `definition` theorem-like environment for these things. `example`
and `algorithm` were added as well.