Add Hayashi's textbook to bibtex database

An earlier commit referenced Hayashi's textbook but neglected to add the record
to the bibtex database.  This commit adds the record.
Revise maximum likelihood text

Rewrites the MLE material to use sentences and paragraphs and to merge it into a single
section.
Move point estimation and probability linear regression material back to document

This commit moves some of the material relevant for linear regression back into
the linear regression document, and out of the probability and estimation
texts.  I'm not sure if it's a good idea or not...
Edit Bayesian material

This commit cleans up and revises some of the material on Bayesian inference.