widgets: use proper method to uncheck checkboxes
widgets: don't subclass widgets.SelectMultiple

So that the infamous "Press Control or Command" help text is not
displayed on django 1.6+.

Fixes #5
treewidget: the db is something we want to have in the repository

So it's easier to develop / test / whatever

And update .gitignore
treewidget: remove unneeded files from test project

Fixes #2
Update README to be hopefully more clear

Fixes #1