fix create_bridge() bug, patch README install recipe, WEAVE_DOCKER_ARGS=...

...''

I set WEAVE_DOCKER_ARGS to a null string to facilitate patching it on install with an inline `sed -i` string substitution so that I (and others) can add a --memory=1gb argument to the build of the weave container, or whatever other arguments folks want to feed it.  

I'm pretty sure that create_bridge() wants to:
+ ip link add name $BRIDGE type bridge 

not:
- ip link add dev $BRIDGE type bridge

at least I could see no joy in trying it the other way.  

I found that the weave shell script failed to `docker pull zettio/weave`, and that I made progress only when I explicitly performed that step manually.  So I added that to the install recipe in the README.md.

I also did a global replace of 'zettio/weave' with $WEAVE_IMAGE and defined that variable at the top of the script, to facilitate maintainance.  

So far I have been able to pull and build an image and run a weave container, and to use weave to wrap docker to run other containers assigning them a fixed IP.  I'm still sorting out how to set the peer for a weave bridge on one docker host to the weave bridge on another docker host.  The documentation seems unclear as to what is expected in that $HOST1 variable, and reading the weaver/weave code reveals no use of a peer attribute in the code, leaving me in doubt as to how and where that argument would be used.