Fix accessing file more than 42mb

Sorry, this is a retry of the original pull#15. I'm doing it proper now with test case in place, as well as doing it on a separate branch.

When doing `File#each` for total chunk size more than 42mb, the following error is observed:

    [2013-04-05 15:29:36] ERROR Moped::Errors::QueryFailure: The operation: #<Moped::Protocol::Query
      @length=144  
      @request_id=4 
      @response_to=0
      @op_code=2004     
      @flags=[:slave_ok]                                
      @full_collection_name="mayday_store_dev.fs.chunks"
      @skip=0 
      @limit=0                                                                                                        
      @selector={"$query"=>{"files_id"=>"515e7d5fc5fe824fdb000001", "n"=>{"$lt"=>9, "$gte"=>0}}, "$orderby"=>{"n"=>1}}
      @fields=nil>                                                                                             
    failed with error 10128: "too much data for sort() with no index.  add an index or specify a smaller limit"
    See https://github.com/mongodb/mongo/blob/master/docs/errors.md

I'm thinking if there is a way to specify query projection for the `has_many :chunks`, this can hack can probably be avoided, but alas, i can't figure out a way to do it.