Move FDL to the end of the document

The license is now last; this will make it easier for people
printing the book for their own use to avoid printing the
8 page license.
Add light gray margin notes for code listings
Change color of code designation

This commit changes the left bar that runs in the margin of any
code from light gray to black.
Reorder some of the linear regression material

This commit makes an 'interpretation of linear regression' 'part'
of the book that includes the material on modeling and on causality.
It sort of serves the purpose of 'advanced topics' on regression.
Merge branch 'master' into rewrite-prob

Conflicts:
	analysis/causality.tex
	analysis/exercises.tex
	analysis/introduction.tex
	core_econometrics.tex
	interpretation/causality.tex
	regression/interpretation.tex
Tweak URL formatting

- Adds a quote-style '<' and '>' to urls.
Fixup
Rewrite Bayesian Metropolis-Hastings material

- Rewrite the text as prose instead of an outline
- Remove R code
- Update makefile