Lookup plugin for etcd

This is a new lookup plugin for CoreOS' [etcd](https://github.com/coreos/etcd).

Example playbook (including deprecated `$ETCD()` invocation):

```yaml

- hosts: localhost
  gather_facts: false
  vars:
  - myname: "{{ lookup('etcd', 'name') }}"
  - mynum: "{{ $ETCD(nr) }}"

  tasks:
  - action: debug msg="the value of name is {{ myname }} (and {{mynum}})"
```