deploy: show full URL to the deployed index.html (fixes #135)

Before, we would show the user that we had deployed into

  server/v1/account/container/path/

This relied on the staticweb middleware being installed and a suitable
web index being configured. We now show

  server/v1/account/container/path/index.html

instead when an index.html file is being deployed. If no index.html
file is uploaded (zapps with a non-standard UI), we show the same URL
as before and the users will have to sort things out by themselves.
deploy: always show where the zapp is deployed

The primary action of this command is to deploy a zapp. This means
that the output telling the user where to find the zapp shouldn't be
seen as info-level log output.