Fix to not include a cookie in TCP mode.

b7098382 added a cookie always, however this only works in HTTP mode.
Remove/disable this automatic addition when we're in TCP mode.
Adding optional image_tag param to docker watcher to explicitly
discover only containers running image_name:image_tag. I've found
this to be quite valuable in the deploy cycle as I can bring up
containers and move the tag tracked in synapse when I'm ready for them to be
discovered. README updated to reflect.
slight typo
Merge pull request #90 from bobtfish/fix_tcp_mode

Fix to not include a cookie in TCP mode.
fix test
updating docker rspec to allow testing multiple different configuration
scenarios
Merge branch 'docker_add_imgtag'