Add Launch{Browser,Market,Email} support on Linux

Modern Linux desktop should implement a xdg-open program, that should open an
URL on the default desktop program.

The documentation of xdg-open says that we should supply xdg-open script on
our installation script for fallback purposes, but I don't think this is
really necessary: anyone using a reasonable recently DE on Linux should have
xdg-utils installed.

More info: http://cgit.freedesktop.org/xdg/xdg-utils/tree/README