Bumped version to 3.0.0

Also added comment to minified file with version number
Merge branch 'master' into jw-three-dot-oh
Merged with master branch
Added note about upgrading to GeoFire 3.x
Improved upgrade to 3.x information
Updated GeoFire README introduction
Changed tests to run on geofire Firebase instead of demo Firebase
Merge pull request #33 from firebase/jw-three-dot-oh

Bumped version to 3.0.0