Node test runner

I've implemented `doctest` runner (issue #34). It can be used like this:

     $ cd ~/doctestjs
     $ npm link
     $ cd ~/your-project
     $ doctest test.js
     ...
     Successes: 10
     Failures: 0

The output is the same as in `node-example.js`. But to make it usable in practice I had to fix several related bugs:

* `require` now works inside evaluated scripts, and also resolves relative imports correctly. This was the most annoying bug to fix. See commits for details.
* changed package version from `0.3` to `0.3.0`, to make npm happy. Even `npm link` didn't work without this change.
* renamed package name from "doctest" to "doctestjs", because "doctest" is already taken on npm. I haven't run `npm publish` yet. Feel free to change the name if you want (e.g. "doctest.js") and publish it.

Commits are somewhat verbose, because I made several unsuccessful attempts to fix `require` bug. I can collapse commits if neccessary.