Add a bunch of HTML tags to the whitelist

Notably, this includes <h[1-6]> and a number of flow-level formatting
tags.
Allow BIDI tags and attributes
Remove yarr_feed_unread class when zero unread

Also switch the default styling to use CSS to add parenthesis around the
count, as they are difficult to remove otherwise (being present in both
the HTML templates and the JS).
1.6 compatibility - defaults has been removed.

django.conf.urls.defaults has been removed in Django 1.6.
Merge pull request #30 from chrisfranklin/patch-1

1.6 compatibility - defaults has been removed.
Merge pull request #31 from twm/unread-count-fix

Remove yarr_feed_unread class when zero unread
Noted merged patches
Merge pull request #32 from twm/keep-headings

Keep headings
Noted merged patch
Fixed entry expiry and changed check_feeds to force db updates
Tidied layout
Clarified reason for upgrade step