fix prefix so that it works in mongo shell

https://jira.mongodb.org/browse/SERVER-445
Merge branch 'master' of https://github.com/CollectionFS/Meteor-cfs-collection

* 'master' of https://github.com/CollectionFS/Meteor-cfs-collection:
  Use correct logic for when calling insert without a callback on the
server; add support for inserting a URL string, which is then
downloaded into a FS.File (done on the server when invoked client-side)

Conflicts:
	common.js