Display progress while pulling images

[andystanton]

Currently if dexec is pulling an image, no indication is displayed. As this can sometimes take a few minutes, it's bad user experience.

This issue has two phases:

1. Print a message to stderr saying an image is being downloaded, specifying the name and tag of the image.
2. Parse the information received from the Docker remote API and either output it in the same way as the Docker client does, or some other minimal way e.g. single percentage rather than per layer percentages.