version: report peer version using --version

By calling the peer's script with --version, we can make `slc --version`
report the version as the peer chooses, instead of only the package
version, like:

    % slc -v
    strong-cli v2.5.3 (node v0.10.26)
    node-inspector v0.8.0
    strong-cluster-control v0.4.0
    strong-registry v1.1.0
    strong-supervisor v0.2.2 (strong-agent v0.4.8, strong-cluster-control v0.4.0)

Prior to this, strong-agent and strong-cluster-control versions would be
unknown.
command: always set CMD so children can use it

lib/loader.js sets env.SLC_COMMAND, but then all children need to
prepend "slc " to this get the original command name (used in usage
messages). node-debug uses the less slc-specific CMD. With this change,
CMD is set to the full "slc whatever" for all sub-commands, so
node-debug is less of a special case.
Merge pull request #142 from strongloop/feature/always-set-CMD-env

command: always set CMD so children can use it
docs: alphabetize sub-commands in usage
docs: add registry sub-command to usage
docs: add build sub-command
debug: update to node-inspector 0.8.0