Stacktrace improvements

There is a lot of nodeunit and async.js noise in the stacktraces... If we can _reliably_ improve the stacktraces, it would make debugging failing tests a little nicer.

I'm a bit worried about messing with something as fundamental as a stacktrace, but it might be worth experimenting with, and could be included as an option to the command line tool at first.

See: http://code.google.com/p/v8/wiki/JavaScriptStackTraceApi

The issue names no files or tests; start at the command-line tool and review the V8 JavaScript Stack Trace API linked in the report. Done means a reliable, experimentally validated reduction of nodeunit and async.js noise, with any initial behavior exposed as an explicit CLI option.

Written by the indexing model from the issue text.