added dropbox script
r-binary-install uses r-install as fallback if not running Linux or FORCE_SOURCE_INSTALL is set
can't use -o when testing with [[
Merge pull request #79 from romainfrancois/master

added dropbox script
Merge pull request #78 from krlmlr/source-install

r-binary-install uses r-install as fallback if not running Linux or FORCE_SOURCE_INSTALL is set