Basic NixOS Private Storage service module

Created by: exarkun

Fixes issue:133

With this you can deploy and configure current HEAD of the Tahoe-LAFS storage plugins integration branch and the ZKAP authorizer storage plugin on NixOS.

Probably many more knobs we want to twiddle (eg location of storage) but this is already pretty big and accomplishes some important goals:

• gets the nixos source layout in place
• demonstrates unit testing and system testing
• moves the docs aside so they're not the primary artifact of this repo
• fixes an unrelated CircleCI regression related to caching :(