Commit UI and Push UI

- When name and email of the user is not set, add two fields to the commit dialog to let the user fill it. Then, it will save them in a pref.
- When github username and password are not set, a dialog will show before pushing the change to give the user an opportunity to fill them.
- Added a temporary git reset settings button in the settings dialog.

@ussuri for ide/app/lib/ui/polymer, ide/app/spark.dart, ide/app/spark_polymer_ui.css, ide/app/spark_polymer_ui.html
@gaurave for ide/app/lib/git/*, ide/app/lib/scm.dart