Add arg which indicates nesting depth of the current event to WireTaps

As a developer using WireTaps, I want to be able to gain a sense of the depth (events triggering other events to fire) in which the WireTap is operating so that I can develop WireTaps that are knowledge of this depth.

If this goes in, I will update (postal.diagnostics)[https://github.com/postaljs/postal.diagnostics] so that its also available. 

This will facilitate logging output that looks like the following:
![screen shot](https://cloud.githubusercontent.com/assets/585619/3818016/9ce80932-1cdd-11e4-9efc-8d8ef36a125e.png)