Raising NotImplementedError in notify_users

Was able to install and setup the plugin for a project, but when testing I get...

_There was an internal error with the Plugin_

```
[ERROR] Plugin(pushover) raised an error during test
Traceback (most recent call last):
File "/home/ubuntu/venv/local/lib/python2.7/site-packages/sentry/web/helpers.py", line 169, in plugin_config
test_results = plugin.test_configuration(project)
File "/home/ubuntu/venv/local/lib/python2.7/site-packages/sentry/plugins/bases/notify.py", line 120, in test_configuration
return self.notify_users(event.group, event, fail_silently=False)
File "/home/ubuntu/venv/local/lib/python2.7/site-packages/sentry/plugins/bases/notify.py", line 62, in notify_users
raise NotImplementedError
NotImplementedError
```

Start with the traceback locations in sentry/plugins/bases/notify.py and sentry/web/helpers.py, especially notify_users and test_configuration. Reproduce the plugin configuration test and trace how the Pushover plugin reaches notify_users; done means the reported configuration test no longer raises NotImplementedError.

Written by the indexing model from the issue text.