Weave 0.8.0

• Align script and image version. When the weave script has a version number, i.e. it is part of an official release, it runs docker images matching that version. Thus the script and image versions are automatically aligned. Unversioned/unreleased weave scripts run the 'latest'-tagged image versions.
• Eliminate dependency on ethtool and conntrack. Instead of requiring these to be installed on the host, weave invokes them via a weavetools docker image that contains minimally packaged versions of these utilities.
• New weave setup command. This downloads all docker images used by weave. Invoking this is strictly optional; its main purpose is to facilitate automated installation of weave and preventing delays in subsequent weave command execution due to image downloading.