Tails torrc

This pull request fixes #282.

I merged the script from https://github.com/micahflee/securedrop-torrc into the securedrop repo, and slightly updated it. I also completely rewrote the readme to make it simpler for people setting it up for the first time.

I also updated the installation docs to make a heavier emphasis on using Tails to connect to the Document Interface. I improved the docs on how to create Tails USB sticks, and told people to create two of them now, one that's air-gapped and one that's used to connect to the Document Interface.

I also modified the GPG key generation docs to generate a passphrase-less key. The security benefits of having a passphrase there are negligible -- if the air-gapped Tails gets malware, it will get the capture the passphrase easily enough. But the usability benefits of not having a passphrase are big. Additionally, now the journalist needs to memorize different passphrases for the two different Tails USB sticks.