Relogit bug fixes and odds ratios

Major changes

• Speed improvements made to relogit. Thanks to @retrography. #88

• Returns relogit weighted case control method to that described in
  King and Langche (2001, eq. 11) and used in the Stata relogit implementation.
  #295

• Odds ratios now returned from summary with relogit models via the
  odds_ratios = TRUE argument. #302