miscellaneous tweaks

[bfgray3]

Thanks for the cool package. It's been very helpful to me. I've done a few different things in this pull request:

• Created data-raw/docs.R to easily track changes in the code that makes R/sysdata.rda.
• Added Depends: R (>= 2.10) to quiet R CMD check.
• Added "a" to The '{PKG}verse' is a selection of packages designed to be loaded in concert.
• Added utils to Imports of the created package since it is used in the source code.
• Imported the pipe from purrr not magrittr since purrr is in Imports and magrittr is not. Slightly different approach than in #9.