[Vala] gksu vapi

[pancake <pancake youterm com>]

I've been looking for the vapi of libgksu with no result. So I decided
to write it down.

Feel free to report changes or commit into vapi/ the ones who have commit access :)

I have opened a bug report for it

https://bugzilla.gnome.org/show_bug.cgi?id=635131

Attached is the vapi and example file.

--pancake