Added a method to record timing events with extra properties

Hi,

I wanted to add properties to a timing event to basically measure what the user did over a session (for example number of games played during an app session).

The only issue I can see is that you might possible pass in a dictionary with a `length` key which would then clash with your `length` key you use for the event length.
I've overcome this by inn `DEBUG` this would crash alerting the programmer to the error, but in release the user set property would simply be overwritten and a `NSLog` outputted.

Cheers,

Rich