store types as json, compatible with browserify

This pull request makes `mime` bundle-able with browserify.

In particular this is useful, because mime is a dependency of request,
which can be used in the browser via browserify, but currently only with an old version of request (before 2.0.0)

All I have done is move `Mime#load` out into a build script,
than you can run after updating the types files.

``` js
node build.js types/* > types.json
```

OR

``` js
npm run build
```

cheers