Cosmetic modifications

- Make the 'final' version use the right fonts
- Add indent to code listings
Tweak wording in regression introduction
Extensive rewrite of model selection material

- Rewrite from outline to paragraphs and text
- Add discussion of model selection problems and Hodges's estimator
- Add simulations
- Move nonlinear and endogenous material to separate chapters.
Version 0.7.0 released

This release has a few interesting changes. The most visible:

* Changes the document class from tufte-book to report (the tufte-book
  class uses way too much italic for some pretty bad fonts).
  Selecting the 'final' option will use good fonts (Bitstream Charter
  and Adobe Source Code Pro) but the default is to use Computer Modern
  to reduce dependencies.

More exciting but less visible:

* The commit embeds the R code for figures and analysis inside the
  latex, then extracts and runs the code with knitr. This removes the
  need for external Sweave files.

  The generated pdfs and macros are included in the repo (which is not
  fantastic practice in general). Think of this as 'reproducible
  research' as an option, rather than by default. The upside is that

      pdflatex core_econometrics.tex

  should work "out of the box."

Content changes:

* The regression modeling section has been rewritten and has material
  on the perils of model selection.