This should do what you want:
Copy and paste this into your favourite text editor:
#! /usr/bin/python
from gtk.gdk import *
w=window_foreign_new((get_default_root_window().property_get("_NET_ACTIVE_WINDOW")[2][0]))
state = w.property_get("_NET_WM_STATE")[2]
maximized='_NET_WM_STATE_MAXIMIZED_HORZ' in state and '_NET_WM_STATE_MAXIMIZED_VERT' in state
if maximized: w.unmaximize()
if w.get_decorations() == 0 :
w.set_decorations(DECOR_ALL)
else:
w.set_decorations(0)
if maximized: w.maximize()
window_process_all_updates()
save in your home folder with the name decoration.py
Add execute permissions:
chmod +x ~/decoration.py
Create a keyboard shortcut:

For the shortcut in the picture - maximize your terminal and type ALT+y to remove the window decoration in the terminal.
