Added a lua function "convert" to convert a userdata value to another gameplay class type.
The purpose of this function is convert a Control to a Button, or PhysicsCollisionObject to a PhysicsRigidBody.
This function will change the meta table of the userdata value to the meta table of the class with the given name.
Updated CHANGES.md
Changed the comment for Matrix.createPerspective to say degrees instead of radians.
Fixed #831
Merge pull request #851 from dgough/next

Added a lua function "convert" to convert a userdata value to another gameplay class type