Move Cesium-hint.js from Apps/Sandcastle/ into a new Apps/Sandcastle/Thirdparty folded
Fix to properly check for 'undefined'

Signed-off-by: andre-nunes <andre.garcia.nunes@gmail.com>
Fix reference to Cesium-hint.js
Exclude new ThirdParty folder from jsHint
Fix path to a simple relative one
Adding andre-nunes to Contributors. #668 was his first merged pull request.
Added GridImageryProvider to CHANGES.md
Merge pull request #639 from andre-nunes/master

 Moved Cesium-hint.js from Apps/Sandcastle/ into a new Apps/Sandcastle/Thirdparty folder
Merge remote-tracking branch 'origin/master' into horizonCullingCleanup