Implement blackbox in Golang (#250)

* Initial release

docs/subversion-tips.md

@@ -0,0 +1,21 @@
+Subversion Tips
+===============
+
+NOTE: This is from v1.  Can someone that uses Subversion check
+this and update it?
+
+
+The current implementation will store the blackbox in `/keyrings` at
+the root of the entire repo.  This will create an issue between
+environments that have different roots (i.e. checking out `/` on
+development vs `/releases/foo` in production). To get around this, you
+can `export BLACKBOX_REPOBASE=/path/to/repo` and set a specific base
+for your repo.
+
+This was originally written for git and supports a two-phase commit,
+in which `commit` is a local commit and "push" sends the change
+upstream to the version control server when something is registered or
+deregistered with the system.  The current implementation will
+immediately `commit` a file (to the upstream subversion server) when
+you execute a `blackbox_*` command.
+