fix bug introduced in add_basic_metadata when trying to eliminate possible redundnant entry
When nexml has no metadata, m <- get_metadata(nexml) returns and empty (length 0) list, and m[dc:pubdate] is NULL. If it is not empty but no element is named dc:pubdate, than m[dc:pubdate] is NA instead of NULL.  Very annoying.  anyway, handles both cases now.
update cleaning routines on Makefile
try adding libgdal-dev to travis explicitly (#61)
Merge pull request #62 from ropensci/mscomm

Edits from Scott
try scott's suggestion of sudo apt-get directly
Merge branch 'devel' of github.com:ropensci/RNeXML into devel
Install some apt packages *before* adding more apt repos.

Fixes #61.
Merge pull request #63 from craigcitro/fixdeps

Install some apt packages *before* adding more apt repos.
define the simmap namespace
update simmap namespaces
support different layouts
writing
updated manuscript
update documentation
restore travis fixes
md copy (built from .Rmd, for easy viewing and text-diffs on github)
including md version, so don't ignore those files
update ignore list to ignore files generated in manuscript builidng
additional clean options: clear cache and figures
cleanup examples