Rename package to pyplot_helpers
Add plot.significance_boxplot...

Add the function:

    def significance_boxplot(plot_context, data_vectors_by_label,
                             colors=None)

From doc-string:

> Plot a box-plot based on the vectors provided.  Annotate the box-plot
> with arrows indicating the _$p$-value_ result from a statistical
> comparison of the corresponding pair of value vectors.
>
> The significance _$p$-values_ are computed using a
> [Wilcoxon signed-rank test][1] and are labelled on the plot using
> either a _red_ or _green_ arrow, pointing to the relevant means in the
> box plot.
>
> A _green_ arrow indicates that the $p$-value is less than 0.05, such
> that the null-hypothesis may be rejected, indicating that the means
> are not statistically likely by sampling from the same distribution.
> If the $p$-value is greater than 0.05, the arrow is _red_.

[1]: http://en.wikipedia.org/wiki/Wilcoxon_signed-rank_test