Calling tidy(input[, callback]) without a callback may throw an error (zepto)

`tidy()` calls `$.trigger()`. Invoking this function without a parameter fails with Zepto.

It fails with a `UNSPECIFIED_EVENT_TYPE_ERR` (1) Dom exception, because Zepto creates an event without a type.

This pull request adds a check to see if `callback` is defined before calling trigger.

(1): http://www.w3.org/TR/DOM-Level-2-Events/events.html