Fixed axis label.
Fixed bug where the plot.mlpsa and mlpsa.distribution.plot would not work if the treatment/control column names were TRUE/FALSE.

This is possibly a bug with ggplot2. Smaller test reveal that ggplot2 has problems with column names of TRUE and FALSE. To workaround this issue we rename the columns before passing them into ggplot2. This has no visible implications for the user since the axis labels are set directly.
Added comment about workaround.