Add modal-open class to body when modals are opened

Given that this is how normal bootstrap works, it makes sense to break React's encapsulation convention.

Not sure what the right way to write a test for this is, but I confirmed that the modal demos on the docs site work the same way as on the bootstrap site.
By the time componentDidMount is executed, document.body should already be defined so this if block should be unnecessary.  the className append can also be shortened to:
```
    document.body.className += document.body.className.length ? ' modal-open' : 'modal-open';
```