Cryptoless Tahoe-LAFS storage server plugin

Fixes: #2 (closed) Fixes: #3 (closed)

This seems to work (that is, it allows the same functionality as native Tahoe-LAFS):

$ tahoe -d client/ create-alias tahoe
Alias 'tahoe' created
$ tahoe -d client/ list-aliases
  tahoe: URI:DIR2:itgd...
$ tahoe -d client/ ls tahoe:
$ tahoe -d client/ put ~/.profile tahoe:.profile
201 Created
URI:CHK:mi3f...:1:1:477
$ diff <(tahoe -d client/ get tahoe:.profile) ~/.profile
$ 

It might make sense to pull the Nix parts of this out to a separate PR. I'm happy to do that if it helps the review process.