gnu: Add r-lattice.

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

gnu: Add r-gridbase.

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

gnu: Add r-plotrix.

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

gnu: Add r-readr.

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

gnu: Rename r-data.table to r-data-table.

* gnu/packages/statistics.scm (r-data.table): Rename variable ...
(r-data-table): ... to this.
[description]: Clarify relation to data.frame.
[license]: Use correct license.

gnu: Add r-devtools.

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

gnu: Add r-rstudioapi.

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

gnu: Add r-git2r.

* gnu/packages/statistics.scm (r-git2r): New variable.

gnu: Add r-httr.

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

gnu: Add r-roxygen2.

* gnu/packages/statistics.scm (r-roxygen2): New variable.

gnu: Add r-brew.

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

gnu: Add r-whisker.

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

gnu: Add r-rversions.

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

gnu: Add r-xml2.

* gnu/packages/statistics.scm (r-xml2): New variable.

Merge branch 'master' into dbus-update

gnu: Add python-statsmodels.

* gnu/packages/statistics.scm (python-statsmodels, python2-statsmodels):
  New variables.

gnu: Add python-patsy.

* gnu/packages/statistics.scm (python-patsy, python2-patsy): New
  variables.

gnu: Add r-data.table.

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

gnu: Add r-chron.

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

Merge branch 'master' into core-updates