Add initial announcements

The new `Announcement` model is going to be used to display updates on
the PyGotham home page. It's a very stripped down example of a blog
post.

References #10
Use reStructuredText for announcements

reStructuredText will allow us to format our announcements without
having to enter a bunch of HTML. This is especially important when it
comes to preserving the whitespace used.

The `rst` filter can be used in other places as well (e.g., talks) as
well.

References #10
Limit supported HTML tags for reStructuredText

Because users are allowed to enter text that will be converted to HTML,
we need to be wary of the tags that are allowed. We don't want anyone to
override styles on the page or inject (harmful) JavaScript. The `bleach`
library can be used to help limit what it's allowed.

`bleach` can be a little too aggressive though, so the list of supported
tags needs to be expanded to included headings and paragraphs.
Add formatting to the announcements

This looks much better than the default stylings.

References #10
Merge pull request #26 from PyGotham/announcements

Announcements