Update configuration

Bump version to 2.0.2, add Guava for demo, remove gradle plugin
management for gwt.xml, update ignores, Vaadin -> RC2
Add date format setting

Add demo for date format + experimental feature for setting the
dateformat for DateStepper. Minor refactoring to DateStepper. Fixes #6
Bump version to 2.1 because API changed

Bump version to 2.1, tweak dist build settings, add all browsers to
gwt.xml