Archivable HTML Report, closes #7

This changeset includes:

* A new `--reports-dir` option for specifying the reports output directory. It defaults to `/tmp`, so `/tmp/zucchini_report.html` will still be there.

* Copying report assets and screenshots into their own directories.

All this actually enables build artifacts archiving, e.g. in Jenkins. I've now got this in my build step:

```
bundle exec zucchini run -s -r "$WORKSPACE" features
```

Plus an "Archive the artifacts" step configured like this:

![screen shot 2013-08-17 at 3 37 06 am](https://f.cloud.github.com/assets/67034/977462/69becac8-06a4-11e3-8c67-e1009a69a3b7.png)

The outcome is:
![screen shot 2013-08-17 at 3 36 18 am](https://f.cloud.github.com/assets/67034/977470/87b87c04-06a4-11e3-97b1-d1d29c66c4a2.png)

Now the "last successful build" link in Jenkins enables a sweet option to download the complete report in a zip file:

![screen shot 2013-08-17 at 4 44 39 am](https://f.cloud.github.com/assets/67034/977481/b309d70e-06a4-11e3-915f-b5fac6d0e019.png)