Allow the action proxy to receive a binary 'code' initializer: a base64 encoded string
of a zipped file. Extract the archive to /action to create a new 'exec'.

Add test, and tweak helper test script to allow for testing from the command line.

Add optional zip to BlackBox exec.
Pull dockerskeleton image to invoker (this will also help with black box actions that are based on this image wrt image caching of layers).

Allow --docker to have code property in exec and assume a specific image.
Add support in CLI and recognize openwhisk/dockerskeleton image in backend to bypass pull.
Add unit test.
Remove obsolete exec field. Refactor to remove duplication.
Add test for blackbox action to confirm empty code when no zip given.