Replace unicode copyright symbol

We're going to move away from unicode to try to make the editing as easy
as possible for other contributors.  This commit starts that process
by replacing the unicode copyright symbol with ASCII or TeX versions.
Make single tex file for entire book

This commit moves the individual parts into a master tex file,
'core_econometrics.tex,' that compiles to a single pdf for the
entire book.  This should make it easier to disseminate the
book and make it easier for contributors to figure out the
internal architecture.
Convert files from xelatex to pdflatex

This commit is a large conversion that aims to make the files
easier for potential contributors to work on.  It converts
the files from using the XeLaTeX engine to build pdfs from
unicode source files to the pdfLaTeX engine and ASCII source
files.
Add more error-tolerant compilation options to makefile

This commit makes the Makefile a little smarter; it checks for
the existence of latexmk and, if it's not there, calls latex
and bibtex repeatedly.

The commit also explicitly deletes files in `burn` and `clean`.

I first saw the latexmk trick in the makefile for *Homotopy Type
Theory: Univalent Foundations of Mathematics* and this is a minor
variation of their approach.
Change marginfigures to figures in bootstrap section

The margin figures were misaligned in the bootstrap section; this commit
changes them to inline figures, but we probably want to switch them
back at some point in the future.
Version 0.6.0: lots of changes to internal structure

This version does a lot internally, but not much in terms of the
content.  The internal changes should make it easier for other
contributors to edit the files.  The changes are:

* Switch the TeX engine from XeLaTeX to pdfLaTeX.  This required
  rewriting all of the unicode symbols in ascii

* Consolidate all four parts of the book into a single document.  This
  should make dissemination and global search/replace easier.

* Update the dependencies listed in the README

* Remove the .md extensions from textfiles where possible

* Speed up the makefile and make it more robust (by checking for
  latexmk)