Histograms

If I've done things correctly, this pull request should reflect me fetching your histograms branch, merging in my prettifications stuff and then requesting that you pull it back into your rstudio/ggvis:histograms.  Looks like this can be automatically merged into your histograms branch.

Note: there are still a few documentation issues and two tests fail because the tests and the specs (as inferred from the code comments) don't exactly match.