GridFS should support WriteConcern option on writes

Namely for GridFSFile.save().

Arguably, might be useful for GridFS ctor as well (along with a ReadPref).

For now one can get the collections and set the WriteConcern/ReadPreference as a default before they use GridFS. The collections will be called <bucket> + ".files"/".chunks" (where the default bucket name is "fs") and one should set the options on both.