Allow specification of Sensu Hipchat 'from' field

This defaults to 'Sensu' in the hipchat handler, which can be
misleading if you have staging and production logging to the same
room.

This enables the following pillar to override it sensibly.

```
sensu:
  notify:
    hipchat_from: 'Sensu Staging'
```
fix typo
Merge pull request #31 from ministryofjustice/hipchat_from

Allow specification of Sensu Hipchat 'from' field