Enable environment keyword at play level

##### Issue Type:

Feature Pull Request

##### Ansible Version:

`ansible 1.8 (devel f4d649bea0)`

##### Environment:

N/A

##### Summary:

Enable environment keyword at play level. This feature enables to setup environment variables at play level.

[This post in ML](https://groups.google.com/forum/#!topic/ansible-project/w3aqRqZ1hIw) also suggests that this feature would be useful.

##### How to use:

```
- hosts: testhost
  roles:
     - php
     - nginx
  environment:
    http_proxy: http://proxy.example.com:8080
```