Build all commands at the same time

In Go it's much more efficient to build several commands in the same
`go build` because the build has to load most of the dependency tree
each time.  Roughly 50% on my machine:

Together (go1.2 on OS X):

    real  0m4.049s
    user  0m8.387s
    sys   0m2.766s

Separate:

    real  0m13.392s
    user  0m12.420s
    sys   0m6.882s

hack/build-go.sh

@@ -31,7 +31,4 @@ if [ $# -gt 0 ]; then
   BINARIES="$@"
 fi
 
-for b in $BINARIES; do
-  echo "+++ Building ${b}"
-  go build "${KUBE_GO_PACKAGE}"/cmd/${b}
-done
+go build $(for b in $BINARIES; do echo "${KUBE_GO_PACKAGE}"/cmd/${b}; done)