Added a UA string test for Kindle

Also added PS Vita so the “silk” detection doesn’t confuse that one.
Merge pull request #1080 from lucbloom/detect-kindle

Added a UA string test for Kindle
Updated readme.
Arcade Physics Body.skipQuadTree is a new boolean that if set to `true` when you collide the Sprite against a Group it will tell Phaser to skip using a QuadTree for that collision. This is handy if this Body is especially large.
Arcade Physics World.skipQuadTree will disable the use of all QuadTrees in collision methods, which can help performance in tightly packed scenes.