Export HTML report with --export

This PR implements #121.

The Teaspoon reports can be exported as a static HTML page. This requires that `wget` is installed.

```sh
teaspoon --export="output_directory"
teaspoon --export # output directory defaults to './teaspoon-export'.
```

To accomplish this, wget downloads from the teaspoon server the HTML and any assets the page needs to load.
The page (by default) is downloaded to ./teaspoon-export/suite_name/index.html.

Specs have been added to cover the new functionality.
This is also manually tested and working in my JRuby app, which mounts teaspoon at a nonstandard route (/web/teaspoon).

Cookie session storage is not available when viewing static html files via file://.
Therefore instead, the HTML reporter uses localStorage when available. If localStorage is not available, it falls back to using cookies.

Please give me any feedback you have. If you're happy with this implementation, I'd love to see this released as a new gem version soon. Thanks!