Add capability to build extension zip-files

Now, typing "make zip-file" will create a standard zip-file for each
extension, with everything necessary, including translations and GSettings schemas.
These files can then be installed with the tweak-tool or uploaded
at extensions.gnome.org

Based on an earlier patch by Jasper St. Pierre.

include.mk

@@ -0,0 +1,11 @@
+extensionurl = http://git.gnome.org/gnome-shell-extensions
+
+# Change these to modify how installation is performed
+topextensiondir = $(datadir)/gnome-shell/extensions
+extensionbase = @gnome-shell-extensions.gcampax.github.com
+
+gschemabase = org.gnome.shell.extensions
+
+uuid = $(EXTENSION_ID)$(extensionbase)
+
+extensiondir = $(topextensiondir)/$(uuid)