Fix a bunch of missteps

- After computing the stats, sort dataframe by x, geoms expect it
  that way, and it is what ggplot2 does
- geom_bar, only set the xtick and xlabels for categorical data,
  prevents numbers with many decimals places
- stat_bin, fix for correctly determining the bins with zero counts
- stat_bin2d, add 'right' parameter
- stat_function, get rid of mapped 'y' value
- util.pop(), to pop dataframes and specify a default value
- six.string_types, fixes Python 3 not understanding unicode