Check if a default mail app is set (for opening mailto: links)

[wellspring]

• Check if a default application is setup for mailto: links (maybe using gconftool-2 --get /desktop/gnome/url-handlers/mailto/command ?)
• Check if the mail application setup as default is installed (= in PATH)

[jsageryd]

Seems gconftool-2 is not available by default on OS X; it would be nice to keep dependencies to a minimum.