Add SCM integration for whisk assets

From @sjfink ""We should consider supporting git repositories as stores for whisk assets, perhaps with a cf-like push/pull model."

ability for the user to instead upload the source code to openwhisk instead tell openwhisk the location of the source code to use (i.e. https://github.com/openwhisk/coolaction.git#)

This requires design on what should be the content of the repo, what does it represent a package, an entity, what type of entity, I guess a manifest file (i.e. wsk.json) can be use to describe what's in the repo, what directory is the action located, etc..