Tweak bibtex key for Billingsley's book
Reorganize linear regression material
Small edits to Bayesian material
Reorganize finite-sample material
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.
Add placeholder file for likelihood asymptotics

I'm going to use likelihoods and their convergence to discuss the LAN-type
efficiency results.