Add support for the `index` option

This PR add an `index` option to the field definition in order to ensure an index in the real MongoDB database.

It is not possible to automate tests of this feature for now, because the core API don't give us an access to the `.getIndexes()` method of the embedded collection. But if you manually test with the query `db.books.getIndexes()` in the `meteor mongo` shell or in a GUI you will see that the index is set.