Fs persistor manager

First version of local filestore persistor manager implementation. I've successfully created, uploaded and downloaded projects using this as well as actually getting done some real-world work on a paper over the past few days with no issues*

Known caveats:
. It won't work on tmpfs file systems (i.e. /tmp on most distributions)
. you need to set `s3.buckets.user_files` and `.template_files`, which is not the most clear of options. I'll be doing a pull req. to shuffle that around later (Will require coordination w/the sharelatex repo).