Step 4: Extract custom component

Custom components are baked right into DartAngular, but when using
Angular with Coffeescript, one needs to pull in the excellent Polymer
javascript library.

Adding Polymer had its own learning curve, though the resulting code
ended up being fairly straightforward. Notable differences between
DartAngular's implementation and Polymer's approach:

* Polymer requires custom components to have a dash in their name,
  DartAngular does not
* Polymer favors more markup to define the component, and no script was
  necessary for this simple example. DartAngular specifies the selector,
  the CSS file, the HTML file, & everything through a Dart file
* With Polymer, the custom component is imported with a `<link>` tag in
  the HTML; in DartAngular it's imported in the Dart code