I don't have GNOME 3.8 (I'm still on 3.2!) but you can help!:
how on earth did you get gnome 3.8 without building it?!?! (unstable repos?)
if you change the metadata.json file (~/.email@example.com/metadata.json) to include "3.8" in the "shell-version" bit and restart gnome-shell, does the extension work (including the prefs)?
(if so, then it is a simple thing for me to add 3.8 support because I only have to update metadata.json, as opposed to the code itself)