Add gridfs bucket option.

http://docs.mongodb.org/manual/core/gridfs/#gridfs-collections
> GridFS places the collections in a common bucket by prefixing each with the bucket name. By default, GridFS uses two collections with names prefixed by fs bucket:

> * fs.files
> * fs.chunks

> You can choose a different bucket name than fs, and create multiple buckets in a single database.

MONGO_CLIENT_API GridFS constructor can take gridfs bucket parameter with name "prefix" and default value "fs".
https://github.com/mongodb/mongo/blob/master/src/mongo/client/gridfs.h#L76