This extension is now obsolete as the gnome calculator provides this functionality since g-s 3.14
gnome-shell-extension-calc
The master branch supports gnome-shell 3.12
Installation
Via extensions.gnome.org
https://extensions.gnome.org/extension/111/calculator/
Manual
git clone git://github.com/eonpatapon/gnome-shell-extension-calc.git
cd gnome-shell-extension-calc
cp -r calc@patapon.info ~/.local/share/gnome-shell/extensions
Restart the shell and then enable the extension.