Ground work for updating auth

[ashcrow]

After upgrading to etcd 2.3.x we started to notice the authentication code was failing. As it turns out there was a change somewhere between 2.1.x and 2.3.x that changes the resulting auth data.

Examples:

• https://coreos.com/etcd/docs/2.1.3/auth_api.html#users-1
• https://coreos.com/etcd/docs/2.3.7/auth_api.html#users-1

This change starts the upgrade process with the following commits:

• Ported @mbarnes auth test fix for acls
• Added version/cluster_version properties to etcd.Client

Hopefully helpful for https://github.com/jplana/python-etcd/issues/210

[coveralls]

Coverage increased (+0.2%) to 88.642% when pulling 1f6e5118dcbbf995680fdefa89b8b761f81ecdff on ashcrow:auth-fixes into 0d0145f5e835aa032c97a0a5e09c4c68b7a03f66 on jplana:master.

[lavagetto]

Very nice, thanks!