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 some real-world work done 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). - will probably want to do this before closing #3.

Tabbing fix brings up a side issue - are there coding conventions on tabs vs spaces + tab widths? I've noticed a mix of the two in the existing filestore code and since coffee is rather sensitive to it, I've been having to run vim w/ `list` on and `expandtab` off and then keep having to do `retabs` on some files...

*Well, no issues with the filestore. Spelling doesn't seem to be running, but I assume that is a config error on my part.