[redis] add redis backend

[hsinhoyeh]

1. imp interface of libkv with redis command and lua script
2. use key space notification for watchXXX/Lock features, so only work for redis version >= 2.8.0

noted: this implementation haven't been battle tested, any further testcases will be appreciated

fixed #116

[hsinhoyeh]

travis failed due to #144, not redis implementation itself

[abronan]

That's a really cool PR, excited to see this landing :)

I'm no longer a maintainer of the library but there are a few people that could help reviewing this. @samalba is a redis guru and will certainly like it.

/cc @sanimej for everything libnetwork related.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "issue-116" git@github.com:hsinhoyeh/libkv.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354269296
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[saracen]

Any idea when this will be merged?