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.
I don't know yet about word `method` here, we use `method` also for the nonlinear optimizer.
ARIMA might be the only case that also has both kinds of method.