Stop `defend`ing (i.e. `Object.freeze`ing).

For performance, basically: http://code.google.com/p/v8/issues/detail?id=1858
Add `longStackJumpLimit`.

- Closes #168, since you can set it to `0` to disable long stack traces entirely.
- Plants the seeds for #144.
Document long stack traces.

This is in particular necessary for people to know how to get performance benefits out of #168.
Merge pull request #170