Always return string from __str__.

If a Section lacked a heading, trying to print it out from the command
line would give a proxy error.
Require unicode-slugify for better slug i18n.

Along with an update to mysociety-django-sluggable, this means the
default slugify function for slug creation will be one that can e.g.
handle CJK names. Thanks to Mark Longair for investigating and tests.
Bring back date on speech detail page.
fix display of speech types/dates on speech page
more translatable speech metadata
Merge branch 'issues/148-lost-speeches'
Merge branch 'issues/361-slugify'

Conflicts:
	setup.py