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.
Merge branch 'master' of github.com:ropensci/RNeXML
update documentation
include system.file based examples in documentation without dontrun
Merge branch 'master' into devel
update with httr-oauth