Rewrite proxy variable material
Rename some chapter files for regression material

Moves regression/endogeneity.tex to regression/causality.tex and moves
the problems to an 'appendix' file.
Add references to Imbens and Wooldridge (2007, 2009)
Add terse IV material

This commit adds a highly incomplete sketch of some material
on instrumental variables.
Edit material on 'systems of equations'

This commit consolidates and edits the material on SUR regression.
Change bullet style

This commit changes all of the 'itemize' list markers into
bullets. The most important change is to remove the long dash,
so that variables don't look negative.
Update changelog
Merge branch 'edit-regressions'
Bump version to 0.7.1