fix typo

disambiguation dilemma between "sauce" and "source"
TST: getting started with test against Stata
REF: wire robust covariances into RegressionResults
ENH/DOC: robustcov, add cluster (not tested), add to docstring
ENH: add Warnings to compare_lr_test and compare_f_test if nonrobust cov
REF: partial extension to ContrastResults for t-test f_test, see #742
ENH: add summary_params_frame to ContrastResult and iolib.summary
REF: bring use_t back, partially, used in t_test
REF: use robustcov in RLM as default, see #1164
REF: add use_t in t_test, add wald_test with use_f
REF: allow title in summary_params, fix typo
BUG: ContastResults.__str__ infinite recursion, fix f_test REF typo
BUG: RLM bcov_unscaled was broken by 61175498ada40061c97caf89b213997d36378f7d
BUG fix import for python 3
TST: add generic tests across models (forgotten to commit)
REF/ENH: self.use_t in summary, additional robust sandwiches, reset df_resid
BUG: regression robust cov, fix and transmit `use_correction`
TST: OLS with cluster robust, first test class (4 failures)
REF: t_test uses model.df_resid not results.df_resid, adjust both
TST: fix tests