Fix broken reprs on notification models

Was getting things like this poking around in the shell:

>>> u.notification_methods.all()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/opt/openduty/venv/local/lib/python2.7/site-packages/django/db/models/query.py", line 74, in __repr__
    return repr(data)
  File "/opt/openduty/venv/local/lib/python2.7/site-packages/django/db/models/base.py", line 423, in __repr__
    u = six.text_type(self)
TypeError: coercing to Unicode: need string or buffer, int found
Respect users notifier ordering

The order notifications were sent did not previously respect the position field
Merge pull request #9 from DazWorrall/notification-ordering

You are awesome, thanks.
Merge pull request #8 from DazWorrall/notification-reprs

Fix broken reprs on notification models
Allow custom retry and escalate period on services
Append the incident ID to email subject lines

Grouped/Threaded conversations based on subject line make the current
email alerts hard to follow - at least they do in GMail!
Remove stray print
Merge pull request #10 from DazWorrall/custom-service-escalation

Allow custom retry and escalate period on services
Increase incident description field to 200 chars
Configurable email subject truncation
Merge pull request #11 from DazWorrall/email-notifier-subject

thanks a lot, merging