Heckman

Added a module to estimate the Heckman selection model using the Heckman 2-step. A test module checks the parameter estimates that this module creates in an example dataset against the parameter estimates that Stata creates. If this happens to merge well into the existing statsmodels code, I'd like to add an MLE estimation method as well. Stata currently has the option to estimate the Heckman model by either the 2-step or by MLE.
add `missing` keyword
missing is handled for endog and exog
but we don't have yet a helper function or automatic `missing` handling for the extra arrays like select_exog
(helper method should be wired up hopefully soon)