set version to 0.2.1

Setting the version to something lower that 1.0 so that we can periodically bump the version when our packages depend on new or revised functionality. I propose that each CRAN release bump the first decimal (e.g. 0.2, 0.3) and that releases in between bump the 3rd decimal. This is what knitr does, whereas shiny has a 4th decimal in case they want to do an e.g. 0.10.1 release. I'm fine with either approach.