formatting changes, v2.0.0+1

updated version number to 2.0.0

added header/footer properties to the panel

added beta warnings

updated pubspec

Merge pull request #62 from akshathjain/dev

Fixed issue where panel listeners would be prematurely called

added defaultPanelState property

updated changelog date

the last two commits had a typo: v0.3.2 -> v0.3.3

added badges