New Callback Plugin: hipchat.py

This pull request adds a new example callback plugin utilizing hipchat.

Required environment variables are `HIPCHAT_TOKEN` to define the API token to use when communication with hipchat.

Additional environment variables are `HIPCHAT_ROOM` and `HIPCHAT_FROM`.

This callback plugin produces output that looks similar to:

```
[1:09 PM] ansible: common: Playbook initiated by sivel against staging
[1:09 PM] ansible: common: 
Tags: all
Skip Tags: None
Limit: None
[1:09 PM] ansible: common: Starting play: Common
[1:09 PM] ansible: common: Playbook Complete
[1:09 PM] ansible: common:
+-----------------------+----+---------+-------------+----------+
|          Host         | Ok | Changed | Unreachable | Failures |
+-----------------------+----+---------+-------------+----------+
|       127.0.0.1       | 4  |    0    |      0      |    0     |
|        staging        | 21 |    0    |      0      |    0     |
+-----------------------+----+---------+-------------+----------+
```

External dependencies are 'prettytable'