Purge bibtex file

Removes bibtex entries that don't appear in the text.  These entries
are left over from earlier versions.
Draft introductory random variable material

Draft of an intro to the probability theory section; it tries to
motivate probabilty measures from densities.  Formally, I guess it's
trying to informally define probabilities from Radon-Nikodym
derivatives w/rt Lebesgue measure.  I also start to rewrite the
sigma-algebra material emphasizing information sets.
Edit introductory probability material

This commit rewords and edits some of the introductory probability
material---random variables, conditioning, etc.