Better error-handling and server-side support

• Exceptions when decoding JSON now throw a real SyntaxError rather than a Plain Old JavaScript Object.
• Loading the SDK and running basic operations (init, identify, track) no longer causes exceptions in non-browser environments (although it doesn't actually send tracking calls over the network). This is useful for allowing mixpanel-js code to run unmodified in contexts like server-side rendering and non-browser tests.