Implement blackbox in Golang (#250)

* Initial release

docs/enable-repo.md

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+Enabling Blackbox on a Repo
+===========================
+
+Overview:
+1. Run the initialization command
+2. Add at least one admin.
+3. Add files. (don't add files before the admins)
+
+The long version:
+
+1. If you don't have a GPG key, set it up using instructions such as:
+[Set up GPG key](https://help.github.com/articles/generating-a-new-gpg-key/). \
+Now you are ready to go.
+
+1. `cd` into a Git, Mercurial, Subversion or Perforce repository and run `blackbox init`.
+
+1. Add yourself with `blackbox admin add YOUR@EMAIL`
+
+1. Commit the files as directed.
+
+That's it!
+
+At this point you should encrypt a file and make sure you can decrypt
+it. This verifies that everything is working as expected.
+
+
+1. Pick a file to be encrypted. Since this is a test, you might want
+   to create a test file.  Call it `secret.txt` and edit the file
+   so that it includes your mother's maiden name.  Just kidding!
+   Store this sentence: `This is my test file.`
+
+2. Run `blackbox file add secret.txt`
+
+3. Decode the encrypted version: `blackbox cat secret.txt`
+
+The "cat" subcommand only accesses the encrypted (`.gpg`) file and is
+a good way to see that the file was encrypted properly.  You should
+see `This is my test file.` 
+
+4  Verify that editing the file works.
+
+To view and/or edit a file, run `blackbox edit --shred secret.txt`
+
+Now encrypt it and shred the original:
+
+```
+blackbox encrypt --shred secret.txt
+```
+
+Now make sure you can decrypt the new file:
+
+```
+blackbox cat secret.txt
+```
+
+You should see the changed text.
+
+Now commit and push `secret.txt.gpg` and you are done!