Add R script to build DESCRIPTION file

* scripts/build_DESCRIPTION.r: Generate content of DESCRIPTION file
  with script for simpler maintenance of the author list.

* DESCRIPTION: Generated from script.

* inst/AUTHORS_libgit2 moved to scripts/AUTHORS_libgit2

* Makefile: Update target roxygen to build DESCRIPTION

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>