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
Merge branch 'hotfix-426-page_teaser' into develop

Conflicts:
	README.rst

Since the addition of the Teaser model requries a schema migration, I
thought it made sense to make it a new major version (0.6), making the
current development changes 0.7.