Polymer: Rename spark_widget files/dir to spark_element_name.*

1. Renamed widget files/dirs from /widgets/lib/spark-element-name to saprk_element_name.
2. Updated Spark app code where using spark_widgets
3. Cleanup /widgets/example/
   - spark-select is removed
4. Added google license on the top of /widgets/lib/spark_modal/spark_modal.html

Fix https://github.com/dart-lang/spark/issues/376, https://github.com/dart-lang/spark/issues/375

Review @ussuri