gnu: Add r-dplyr.

* gnu/packages/statistics.scm (r-dplyr): New variable.

gnu: Add r-r6.

* gnu/packages/statistics.scm (r-r6): New variable.

gnu: Add r-testthat.

* gnu/packages/statistics.scm (r-testthat): New variable.

gnu: Add r-crayon.

* gnu/packages/statistics.scm (r-crayon): New variable.

gnu: Add r-memoise.

* gnu/packages/statistics.scm (r-memoise): New variable.

gnu: Add r-pryr.

* gnu/packages/statistics.scm (r-pryr): New variable.

gnu: Add r-codetools.

* gnu/packages/statistics.scm (r-codetools): New variable.

gnu: Add r-microbenchmark.

* gnu/packages/statistics.scm (r-microbenchmark): New variable.

gnu: Add r-knitr.

* gnu/packages/statistics.scm (r-knitr): New variable.

gnu: Add r-yaml.

* gnu/packages/statistics.scm (r-yaml): New variable.

gnu: Add r-markdown.

* gnu/packages/statistics.scm (r-markdown): New variable.

gnu: Add r-mime.

* gnu/packages/statistics.scm (r-mime): New variable.

gnu: Add r-highr.

* gnu/packages/statistics.scm (r-highr): New variable.

gnu: Add r-formatr.

* gnu/packages/statistics.scm (r-formatr): New variable.

gnu: Add r-evaluate.

* gnu/packages/statistics.scm (r-evaluate): New variable.

gnu: Add r-bh.

* gnu/packages/statistics.scm (r-bh): New variable.

gnu: Use cran-uri in source expressions.

* gnu/packages/statistics.scm (r-colorspace, r-dichromat, r-digest,
  r-gtable, r-labeling, r-magrittr, r-munsell, r-rcpp, r-plyr, r-proto,
  r-rcolorbrewer, r-stringi, r-stringr, r-reshape2, r-scales, r-ggplot2,
  r-assertthat, r-lazyeval, r-dbi): Use cran-uri in source expressions.

import: cran-sxml->sexp: Use cran-uri in package expression.

* guix/import/cran.scm (cran-sxml->sexp): Use the cran-uri procedure in
  the generated package expression.

build: Add cran-uri procedure.

* guix/build-system/r.scm (cran-uri): New procedure.

gnu: Add shotwell.

* gnu/packages/gnome.scm (shotwell): New variable.