Set window title to Spark in Polymer UI (was SparkPolymer)
Freeze the default UI to be Polymer + hide "Switch UI" menu by default

1) The last used UI was persisted before. Now the default UI can be enforced locally by modifying the value of DEFAULT_SKIN_ in background.js.
2) The "Switch UI" can be temporarily brought back by modifying the value of CAN_SWITCH_SKINS_ in background.js.
Rename "skin" -> "ui" in variable names
Merge pull request #570 from ussuri/polymer-switchover

Polymer switchover