Initial version of spark-dialog

More-or-less works but is badly stiled (turned off Bootstrap)
Initial version of spark-dialog-button
Change default style to flat
Standardize overlay toggling attribute name ('overlay-toggle' -> 'overlayToggle')

Didn't change the usage in spark_polymer_ui.html, since that's going to be all rewritten.
Undo debugging artifacts
Address review comments
Correctly distribute main body content and spark-dialog-buttons + justify buttons to right
Merge pull request #2176 from ussuri/spark-dialog-2

Introducing <spark-dialog>