Given an array of stack frames, convert to V8 stack trace `stack` prop

[niftylettuce]

Right now output from toString() on a stackframe looks like this:

setTimeout(function()@http://localhost:3000/js/core.js:16:9

Which doesn't match normal output from err.stack prop, e.g. https://github.com/nodejs/node/blob/6ce87c027dc2a16e1b8d85c753b52270ae0c6054/lib/internal/errors.js#L43 (has "at ...")

It would be nice to have a method where we can convert an Array of stackframes to match Node's err.stack.

[niftylettuce]

Closing in favor of #22