Test cases

Beginning of test case infrastructure using vows. The current tests will just generate HTML files for each of the example specs as well as do some basic sanity checking.

To see the tests in action, run:

```bash
git clone https://github.com/mprudhom/vega.git && cd vega && npm install && VEGA_TEST_OUTPUT=${TMPDIR} make test
```

... and then examine the .html files in TMPDIR. Verified on OS X (Mountain Lion) and Ubuntu  (Quantal Quetzal).