Release version 0.4

The most important change is the rename from "sb" to "gob". As such, all
binaries now have a "gob-" prefix instead of a "sb-" prefix. Other than
that, this is mostly a cleanup release, including initial documentation,
improved tests and some minor cleanups.

Furthermore, the block storage now has a version associated with it.
This makes sure that gob will not try to write to block stores which
have a different version, especially in the case of
backwards-incompatible changes.