Hacked MOAIScriptNode to check whether isAllowedToCallOnUpdate is set before pulling links in onDepNodeUpdate.
Accidentally removed type-checking on the second argument in MOAIScriptNode's _setCallback.  Oops.
Merge pull request #22 from mindsnacks/feature/script-node-clear-callback-hack

Hacky hack to stop MOAIScriptNode from crashing Moai.