ZPT fragment support

Implements support for defining theme fragments using ZPT that can be included into the theme via ``href`` support. Some key considerations:

 * Fragments are public: just like you can traverse to a theme resource directory and read the static HTML and other resources, you can traverse to a theme fragment; however...

 * Themes execute in Restricted Python, so the current user's security applies

 * Fragments are only available for the currently active theme

With this model, we avoid fragments becoming a generic view registration mechanism. There's no configuration, for instance, and fragments are tied directly to themes.