Add a docker commit step to the docker builder that runs instead of export if export_path not present
Add a docker tag post processor
Add a docker save post processor
Add Stderr capture to Commit and TagImage calls.
Merge branch 'feature/docker-image' of github.com:andytson/packer into andytson-feature/docker-image

Conflicts:
	config.go
Merge branch 'andytson-feature/docker-image'
builder/docker: change commit to the opt-in, special case build steps

/cc @andytson - Wanted to CC you in here so you could see some changes I
made.

First, I changed commit to opt-in, so you set "commit": true to get
commit behavior. This simplifies the logic a bit. Then, I removed the
skipping for StepExport and StepCommit and put that into the Builder
itself. This simplifies those steps (limits abstraction leakage).
Otherwise, everything looks great!