Provide a way to bypass the use of sudo

It's not necessary to be root to run "docker" commands, if your
user is in the "docker" group.
Don't use /tmp for big files

/tmp is on tmpfs on some systems, so it's better to use /var/tmp
for large files.
Comment typo fix
Merge pull request #102 from dpw/makefile-tweaks

Makefile tweaks
more portable bridge creation
using the documented 'ip link add' syntax

Thanks to @hesco for the suggestion.
refactor: extract weave docker image name

Thanks to @hesco for the suggestion.
make it clearer what $HOST1/2 refer to

based on suggestion by @hesco
document WEAVE_DOCKER_ARGS
Added link to weave blog