better remove api on GridFS and GridFile

The current api to find files in GridFS is great: one easy by filename, and one extensive by BSONObj query.
However, to remove files we only have "removeFile()" by filename.

It would be a great (and easy to implement) addition to have a BSONObj query api for remove (optionally with a skip and limit too, but it's less useful, and should also be added to the find method).

Also, there is no remove() method on the GridFile class, this could be really useful too.

Currently when multiple files have the same filename we have to manually delete the documents in fs.files and fs.chunks collections, like it's done in GridFS::removeFile().