Bump to 0.7.16 dev.
Allow extensions to specify their list of app directories.

Extensions can define an 'apps' list as part of their class definition.
When loading, all items listed in apps will be added to INSTALLED_APPS,
and when unloading, those will all be removed.

If an extension does not provide 'apps', then this falls back on the
default behavior of using the app containing the extension class.
Improve the look and feel of extension configuration.

Extension configuration did a lot of the same work that siteconfig did,
but worse.

It didn't fit in with the rest of the templates at all, making it stand
out noticeably.

It was lacking features from siteconfig's SiteSettingsForm, such as save
blacklists and fieldsets, requiring the extension author to work around it.

It also didn't have any notification of when settings were saved. It
looked like clicking Save did nothing. What's worse is that hitting
reload on the page would prompt to resubmit the form data, since we
didn't redirect.

Extensions now reuse all the siteconfig code for this. An extension's
Settings object conforms to the siteconfig API for setting/getting data,
and all the templates and form code is mostly siteconfig code.

Now, configuration fits in with the rest of the UI, and isn't so buggy.
Add a URL link for extension authors, and make the page more stylable.

The extensions list page now links the author to the extension's URL, if
one is available.

The name, version and author are now grouped together in a div that can
be styled. The page also has a class so that decisions can be made
involving the overall style of the extensions list page.

There's also now an author_url field (mapping to 'Author-home-page')
that extensions can provide in Extension.metadata for specifying the
URL for the author's site, for the link. By default, this will point to
the extension's URL (which is generally the project page for the
extension).
Allow the use of icon class names instead of image URLs in places.

The datagrid columns and inlineEditor now accept class names
representing icons for column and edit icons. These can be used instead
of URLs, for applications that use spritesheets.
Escape field values in datagrid cells.

Datagrid cells would pull data from a model and render it as-is. This
could lead to issues when using certain characters. We now escape
everything coming from the database.

If a custom renderer is provided, it's still expected that it returns
valid, safe HTML.
Merge branch 'release-0.6.x'