Titanium constants

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These are several useful constants in Alloy.

OS_IOS True if the app is running on iOS
OS_ANDROID True if the app is running on Android
OS_MOBILEWEB True if the app is running in mobile web
ENV_DEV True if the app is running in the simulator
ENV_TEST True if the app is running on a device
ENV_PRODUCTION True if the app has been built for distribution

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