Reorganize linear regression material

This commit consolodates the linear regression material and splits
it into two separate parts at the end of the book. It also adds some
links to the Inbens and Wooldridge (2007) online lectures.
Reorganize probability and statitistics material

* Bayesian inference is moved to the probability section so that
  we can do something interesting with data quickly
* Asymptotics section has been integrated with the rest of the
  material; CLT and LLN are now a single chapter in the probability
  section. The other material is in statistical inference, but
  needs to be better integrated.

Conflicts:
	.gitignore
	Makefile
Clean up the table of contents

Some minor changes to numbering and formatting in the
table of contents.