publish to npm

Since the tests all run in node now without a requirement on ruby, why not publish this to npm? It's pretty easy! The first thing this package needs is a `"main"` field, which I've already included in this pull request. Also the `jasmine` name is currently available so you should claim it!

After merging this pull request:

1. If you don't already have an npm username, first create one with:

```
npm adduser
```

and npm will ask you some questions to set up your account.

2. From the jasmine project root, do `npm publish`.

Now everybody can `npm install jasmine`!

If you want to add more maintainers, do:

```
npm owner add USERNAME jasmine
```

That's it!