Add exception handling to Fuse.Timer

Currently, Fuse.Timer just rethrows any errors when executing the callback. I'm thinking of adding a new option (a la Ajax.Request) for specifying a callback if an exception is encountered (if it's not specified, the error will just be rethrown):

//Old syntax
new Fuse.Timer({
    callback: function(timer){ ... },
    timeout: 10
});
//New syntax
new Fuse.Timer({
    //`onTimeout` seems more consistent with `onException` than `callback`...should we change it?
    onTimeout: function(timer){ ... },
    onException: function(exception, timer){ ... },
    timeout: 10
});