Add a teaser model

Teaser is a per-language, short description of a page.  I had to
override the ``PageAdmin`` class for Django CMS pages to include the
teaser in the Page admin.

Addresses #426
Added __unicode__ method, docstrings to Teaser

Add a __unicode__ method and some docstrings and comments to the Teaser
model and the custom admin.

Addresses #426
Update changelog and docs for teaser feature