scrub out all plyr examples

[brownag]

Can result in some difficult-to-diagnose issues when, for example, library(plyr) is called after library(dplyr).

A good example of a problem is plyr::summarize() does not respect grouped data.frames, which can then break a number of examples used in the book. While using :: syntax is a solution, we do not currently use it consistently and it would be better if we provided a more consistent approach to suggested packages.

Need to look thru old final projects and examples outside the book and eliminate all uses. There is really no good reason to use plyr if you are a new R user, and it is potentially confusing for folks not familiar with the history as the package names are very similar.

Currently there is nothing current in book 1 that uses it, but I see several instances in book 2.

[dylanbeaudette]

Agreed. I'll remove / refactor all of the remaining references to plyr, seems like it is just the Appdx material.