Implement blackbox in Golang (#250)

* Initial release

docs/dev-code-overview.md

@@ -0,0 +1,36 @@
+Code Overview
+=============
+
+Here is how the code is laid out.
+
+TODO(tlim): Add a diagram of the layers
+
+```
+cmd/blackbox/   The command line tool.
+             blackbox.go   main()
+             cli.go        Definition of all subcommands and flags
+             drive.go      Processes flags and calls functions in verbs.go
+                   NOTE: These are the only files that are aware of the
+                         flags.  Everything else gets the flag data passed to it
+                         as a parameter. This way the remaining system can be
+                         used as a module.
+
+pkg/box/        High-level functions related to "the black box".
+        verbs.go       One function per subcommand.
+        box.go         Functions for manipulating the files in .blackbox
+        boxutils.go    Helper functions for the above.
+
+pkg/bblog/      Module that provides logging facilities.
+pkg/bbutil/     Functions that are useful to box, plug-ins, etc.
+pkg/tainedname/ Module for printing filenames escaped for Bash.
+
+models/vcs.go        The interface that defines a VCS plug-in.
+models/crypters.go   The interface that defines a GPG plug-in.
+
+pkg/crypters/   Plug-ins for GPG functionality.
+pkg/crypters/gnupg   Plug-in that runs an external gpg binary (found via $PATH)
+
+pkg/vcs/        Plug-ins for VCS functionality.
+pkg/vcs/none        Repo-less mode.
+pkg/vcs/git         Git mode.
+```