Turn off HTML escaping in plain text email

Django automatically escapes strings for HTML in its templates. The
autoescape tag is used to avoid HTML escaping for plain text email.
Add Unicode data to unit tests

The only risky area that is reasonably testable is the text processing
in digest.py, so tests are added there. System-level tests should be
added to test interaction with other services.
Merge pull request #9 from edx/gprice/unicode-data

Add Unicode data to unit tests