Implement blackbox in Golang (#250)

* Initial release

docs/expired-keys.md

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+Replacing expired keys
+======================
+
+If someone's key has already expired, blackbox will stop
+encrypting.  You see this error:
+
+```
+$ blackbox_edit_end modified_file.txt
+--> Error: can't re-encrypt because a key has expired.
+```
+
+FYI: Your repo may use `keyrings/live` instead of `.blackbox`. See "Where is the configuration stored?"
+
+You can also detect keys that are about to expire by issuing this command and manually reviewing the "expired:" dates:
+
+    gpg --homedir=.blackbox  --list-keys
+
+or... list UIDs that will expire within 1 month from today: (Warning: this also lists keys without an expiration date)
+
+    gpg --homedir=.blackbox --list-keys  --with-colons --fixed-list-mode  | grep ^uid | awk -F: '$6 < '$(( $(date +%s) + 2592000))
+
+Here's how to replace the key:
+
+- Step 1. Administrator removes expired user:
+
+Warning: This process will erase any unencrypted files that you were in the process of editing. Copy them elsewhere and restore the changes when done.
+
+```
+blackbox_removeadmin expired_user@example.com
+# This next command overwrites any changed unencrypted files. See warning above.
+blackbox_update_all_files
+git commit -m "Re-encrypt all files"
+gpg --homedir=.blackbox --delete-key expired_user@example.com
+git commit -m 'Cleaned expired_user@example.com from keyring'  .blackbox/*
+git push
+```
+
+- Step 2. Expired user adds an updated key:
+
+```
+git pull
+blackbox_addadmin updated_user@example.com
+git commit -m'NEW ADMIN: updated_user@example.com .blackbox/pubring.gpg .blackbox/trustdb.gpg .blackbox/blackbox-admins.txt
+git push
+```
+
+- Step 3. Administrator re-encrypts all files with the updated key of the expired user:
+
+```
+git pull
+gpg --import .blackbox/pubring.gpg
+blackbox_update_all_files
+git commit -m "Re-encrypt all files"
+git push
+```
+
+- Step 4: Clean up:
+
+Any files that were temporarily copied in the first step so as to not be overwritten can now be copied back and re-encrypted with the `blackbox_edit_end` command.
+
+(Thanks to @chishaku for finding a solution to this problem!)
+