Importing mangler here does not work

If we're running twibuilder in a plugin with a mangler
an error happens: KeyError of tiddlywebplugins on sys.modules
_because_ not tiddlywebplugins have been imported yet: the namespace
does not exist.

Therefore what needs to happen is that twibuilder is tested
via code, never by running the script (which does next to nothing
anyway).
release 0.1.2