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.
this is python 3 only, I guess, according to the TravisCI failure.
I think you could use pandas directly to load the data. 
Later we will include the csv file, if it is public domain or license compatible, I assume it is.