GridStore based reimplementation

@aldeed, @raix:  You guys are welcome to pull this into a devel branch (because of the backward compatibility issue https://github.com/CollectionFS/Meteor-cfs-gridfs/issues/2#issuecomment-36412509).  It would be straightforward to add `getStream` and `putStream` methods, which would be a big improvement given that currently it requires 2*filesize bytes of memory to get/put. This is the also the case in the existing implementation due to the Buffer <--> Uint8Array conversion.