version date      description
0.1     07/19/12  The first version on CRAN
0.2     09/27/12  The second version.  Fixed a bug in the ATE CBPS.
0.3  	01/18/13  The third version.  Updated the ATE variance estimate.
0.4     03/30/13  Adds function for fitting marginal structural model.  Fixed standard error in cross-sectional case.
0.5 	05/07/13  Adds function that handles three- and four-valued treatments.