Add notes on data summarization to point estimation doc

This commit adds notes on a few basic summarization principles: the
idea behind sufficient statistics, and a brief statement about the
likelihood function, to the beginning of the estimation document."
Add new LaTeX command for 'dx' and update probability text

Adds a 'dx' macro for integration as well as a few others.
Rewrite notes on method of moments

Writes up the method of moments notes as actual prose, not a half-baked
outline.
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.
Reorganize finite sample material, but leave in intervals
Restore confidence interval material to estimation document

This commit restores the old confidence interval material that was
accidentally erased earlier.
Edit interval estimation text

This commit rewrites the existing interval estimation material in
sentences and paragraphs.  It also adds a brief discussion of
optimality.
Edit optimality material in estimation document

This commit rewrites the optimality material to use paragraphs
and sentences instead of a terse outline.
Rewrite and move some linear regression material

This commit rewrites some of the finite-sample linear regression material
to use words and paragraphs, etc.  It also moves the algebraic and statistical
(finite-sample) properties of linear regression back with the rest of the
finite-sample material than they do on their own.