Notification email copy and formatting

This pull request adds the following:

* Updated the notification text
* Added a new html version of the notification email that we can work on.
* A new http://localhost:5000/dev/emails endpoint for previewing the emails.

I've been trying to add tests for the html emails to ensure that everything is passed correctly to the mailer. But it turns out that the whole test suite is barely functioning, as only a partial annotation is created in each test, the `render` method blows up every time.

I've tried to fill this in, but I'm running into issues with the template handlers not being registered.

> No such renderer factory .txt

It's not helped by this except handler catching everything so you never see _any_ exceptions from failed email generation.
https://github.com/hypothesis/h/blob/notification-email/h/notifier.py#L60-62 

Could someone give me a few pointers here?

Here's the current email as it stands:

![screen shot 2014-08-12 at 13 14 00](https://cloud.githubusercontent.com/assets/47144/3889614/c9f366cc-2211-11e4-84ad-cdfcfb52d494.png)

Not sure that this is a) the right way to go about this and b) if so, if there is already an exception class I can use.