Add command to make Emacs window fullscreen.

Default keybinding is F11. The command follows freedesktop standards,
so it should work on most desktops, at least in GNU/Linux systems.
Restrict fullscreen feature to X-based systems.

Recent Emacs in OSX already comes with fullscreen support.
Merge pull request #359 from haplo/fullscreen

Add command to make Emacs window fullscreen