XBox 360 Support for Mac OS

Adds support for XBox 360 controllers to the current Mac OS gamepad
support.  Also adds null checks to one of Gamepad's constructors.
Change header and lib search paths in xcode project.

Relative paths to external-deps and src folders were assuming that the
repo was cloned to a folder named "Gameplay", and not e.g.
"Gameplay-Next" or "MyGameplayFork".
Fixed userData pointer not cloned when cloning a node.
Merging with upstream/next
Merge pull request #844 from ablake/next

XBox 360 controller support for Mac OS
Merge pull request #845 from WolfgangD/next

Fixed userData pointer not cloned when cloning a node.
Changed ScriptController so you can execute a lua function that is in a table.
Previously it was only possible to execute a global lua function using ScriptController::executeFunction.
I've also changed many of the ScriptController getters so you can get numbers and strings that are stored in tables.

_script->getString("tableA.tableB.str");
_script->executeFunction<void>("tableA.func");
Merge pull request #850 from dgough/next

Changed ScriptController so you can execute a lua function that is in a table