Rewrite introduction to asymptotic material

Rewrites the introductory L_p material using sentences and paragraphs.
Rewrite convergence in probability material

Revises and streamlines the convergence in probability and WLLN
material.
Rewrite almost sure convergence material

This commit cuts the computer material for almost sure convergence
and rewrites the text in paragraphs and sentences.
Start CLT section
Move almost sure convergence to probability document

Almost sure convergence plays very little role in the first year
econometrics sequence, so I think we'll just mention it (and the SLLN)
in passing towards the beginning and focus on Lp and convergence
in probability in the asymptotics document.