glock cmd: support for building and updating Go commands

Leading lines in a GLOCKFILE like:
   cmd code.google.com/p/go.tools/cmd/godoc
   cmd github.com/robfig/glock

will have their dependencies included in the calculation.

The commands will be built (if necessary) during "glock sync".

New commands will be installed by "glock apply", and all commands will be
'go install'ed if any package versions are updated.

"glock cmd" can be used to record new commands in your GLOCKFILE. It will also
add the dependencies of those commands to the list of revisions.