Add an alternative version of Polymer-based UI + enable switching between UIs.

Not checking in the full up-to-date Polymer-based UI yet to simplify reviewing.
Reuse spark.css in spark_polymer.css
Small non-functional changes/fixes
Go back to calling webkitStartDart(): doesn't cause problems anymore

...Probably after some fixes in Polymer 0.8.5.
Fixed library name in pubspec.
Removed spark_polymer_bootstrap.js
Conform to the latest breaking changes in Dartium and fixes in Polymer Dart

1) Single *.dart script tag per HTML is required by Dartium now.
2) Polymer now allows only 1 main() per app, and that used to be taken by init.dart.
3) However, Polymer also fixed an bug around that, and now <script src='init.dart'> can be replaced with initPolymer() in the app's own main().
Added the new spark_polymer.html to dart2js compilation step.
Add spark_polymer.html to the deploy step
Merge pull request #82 from ussuri/ui-in-polymer

Add an alternative Polymer-based UI + persist window geometry between launches