Always fire the script installation callback asynchronously.
Always fire the script installation callback asynchronously.

This fixes scripts being injected twice when all @requires are removed.

Martii/greasemonkey,HasClass0/greasemonkey,Ventero/greasemonkey,the8472/greasemonkey,HasClass0/greasemonkey,Ventero/greasemonkey,the8472/greasemonkey,Martii/greasemonkey