use either /usr/bin/open or /usr/bin/xdg-open for sending e-mail when RStudio is active
extract function email_browser, use Sys.which
use early termination instead of deeply-nested if-s
Merge pull request #443 from krlmlr/email

use either /usr/bin/open or /usr/bin/xdg-open for sending e-mail when RStudio is active