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.
Add class hooks for styles to all directives
Hide model list when empty
Add styles to demo
Style dropdowns
Add Dropdown service for tracking multiple drowndowns
Close drowpdown on click outside
Close dropdown on Esc
Add disabled state to options list
Add custom templateUrl
Namespace templates module
Always access model and options from $scope

I was planning on trusting the references to the options and model, but
this means I lose them if they get replaced in user-land.
Add selected attribute for simple list of model
Watch options list for changes
Add custom title
Watch template-url attribute for changes