Fixup build tasks
Add basic multiselect

We have to immediately leave <select> behind so that we'll be able to
control the look and feel in the future. With it we lose ng-model
and ng-options.
Add Choice service to expand option lists

Now we can accept:

* Simple lists ['val1', 'val2']
* Values only [{value: 'val1'}, {value: 'val2'}]
* Full lists [{value: 'val1', label: 'label1'}, {value: 'val2', label: 'label1'}]

Extra properties will be ignored, but brought along for the ride to the
model in case the user has their own information they'd like to keep.
Add Sass compile
Add dropdown control

Here we're implementing a simple internal directive to power the
dropdow. This might be overkill, but it could easily be subbed out later
or replaced by a user's custom template.