[Greasemonkey] Bait and switch script installation

[Julien Couvreur]

I added the threat to the Greasemonkey threat model entry on the wiki
( http://dunck.us/collab/GreaseMonkeyThreatModel ).

I'd like to take this occasion to also recommend to anyone hosting
Greasemonkey scripts with some bandwidth constraints to turn on HTTP
compression (see mod_gzip for Apache webserver) and set the expiration
date headers (see mod_expires).
This way you save bytes transfered in a given request (compression)
and you avoid un-necessary requests (caching).

Cheers,
Julien