Introducing email notifications to page owners.

This commit introduces a new email-notification template, called: DocumentOwnerTemplate and a new consumer for our AnnotationEvent domain_notification

Description:
- The code is in a separate file, called: domain_mailer.py + 2 template files for the notification template.
- These 3 files can be always moved out from our h project and use h as an import.
- Only this part of the code uses requests, and BeautifulSoup4, so requirements will be obsolete once we move this component to a separate repo.
- For every newly created annotation we download the web page where it is created, and if needed we reparse it to look for link tags with rel='reply-to' property, to identify the document owners.
- If the email's domain match the pages domain: we send an e-mail notification to that page owner
- The DocumentOwnerTemplate has its subject and body in a separate txt template, as usual, so easy to change it.
- Currently we're not doing any caching, this will be subject to later performance improvement PRs
Add BeautifulSoup and requests to setup.py
Merge pull request #1085 from hypothesis/email-document-owner-notifications

Introducing email notifications to page owners.