allow installation with npm

This continues the work of @caseygrun by adding a name for this package that has not yet been taken in the npm registry.

It would be really cool to get this published to `npm`!

The steps:

* land this pull request
* publish to npm
```
cd FileSaver.js
npm adduser
npm publish
```

then we can install using `npm install browser-filesaver` and use [browserify](http://browserify.org) to bundle this up with the rest of the project!