Bug fixes and enhancements

A bug fix and improvements in this release:

• Fixed issue #4 where contentgroups with differing height interfere with closing the nav.
• Support root nav item as a link.
• Added new background property to ContentGroup to support different content group background colours.
• Added new contentTop property to SiteNav so the distance between the nav and flyouts can be configured.

Off screen detection and correction and better arrow animation

• Added view port detection so when flyouts are off screen, they get corrected and rendered on screen.
• Completely refactored arrow component so it's decoupled from the content div and improved its animation.