Election date filtering widget.

Add method to retrieve ordered list of all elections in an
Elections collection.

Add a view that leverages the [jQuery UI
slider](http://jqueryui.com/slider/#range) to set a range of
dates for filtering the available elections.

Wire up some Backbone events to connect the filter changes to
re-rendering the elections table.

Addresses https://github.com/openelections/core/issues/165