Added the design documentation for Cox & Sandwich

Conflicts resolved by Feng, Xixuan (Aaron):
	doc/design/design.tex
	doc/literature.bib
	src/ports/postgres/modules/stats/cox_prop_hazards.sql_in
Merge pull request #16 from madlib-internal/old_cox_design_doc

Added the Design documentation for Cox & Sandwich (Authored in summer, 2012)
ARIMA: Update online help to not use _diagnostics