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

Fixed #848 
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.

```lua
-- Function signature:
-- @param object    The gameplay userdata object.
-- @param className The string name of the class to convert to.
function convert(object, className)

-- Example:
local launchButton = form:getControl("launch")
if launchButton ~= nil then
  convert(launchButton, "Button")
  print("text: " .. launchButton:getText())
end
```