Add support for memo backend.

[mnottale]

No description provided.

[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 "memo-backend" [email protected]:infinit/libkv.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

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

[Dimrok]

Hi @mnottale.

Could you add some tests, like the other backends please (e.g. zookeeper_test, etcd_test, etc.).

[Dimrok]

Hi @mnottale.

seems good but could you create different PRs for each separate problems you fixed, because:

• only 5 commits are related to memo integration in libkv (15fd92e, 5d43b1e,
  2cc4745, a7a9e2c, 79fed48)
• 2 are related to the good health of the test suite (6fa608e, 18a2759)
• the 3 lasts change travis configuration (3a5aafd, 10523b1, 14f8215)