chore(cmd/doozer): move into the src directory

moving this into the src directory makes it easier to hack on since,
with a proper GOPATH set, you can simply say `go build util/doozer`.

I can't use the package prefix cmd because go uses that for itself
apparently (see $GOROOT/src/cmd)

Also add a hacking section and note on how to build the doozer util
fix(Makefile): build the doozer command by default

most people checking out this source will want to build the doozer
command to start playing around with it. Make that the default target.