MixPanel: handle SecurityErrorEvent and propogate it to callback

This handles SecurityErrorEvent events that otherwise cause an error (Error #2044: Unhandled securityError) for the user of the library and ultimately the browser user. I couldn't figure out a way to handle this without a change to MixPanel. On the other hand, the change is very small. I chose to give the callback the whole event object since it helps with diagnostics. I tested under windows building with FlashBuilder 4.7. Sorry for not splitting the reordering of the imports to a separate patch, laziness.