Extract client from cli

This moves the (http) client from commands.go to it's own package to make it possible to reuse the same facilities when writing 3rd party docker api clients. In general this could use some refactoring, but I've tried to keep the change as small as possible for now. Anyway, I think it's a good start. As "keeb" in IRC said:

        12:38 < keeb> fish_: absolutely, that sounds great. part of the goal is to /eventually/ make the remote API just a plugin for Docker. Having a client lib in place that is derived from the canonical/supported code seems straightforward