Z3c.pagelet

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* Allow use of ``z3c.pt`` using ``z3c.ptcompat`` compatibility layer.

* Add support for context-specific layout and content template lookup,
using (view, request, context) discriminator. This is compatible with
context-specific templates introduced in z3c.template 1.2.0.

* Don't do rendering in pagelet's __call__ method when request is a redirection.

* Add sphinx-based HTML documentation building part to the buildout.

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* Added a `form.zcml` which can be included to have a template for
``PageletAddForm``, ``PageletEditForm`` and ``PageletDisplayForm``.

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* Added ``update()`` and ``render()`` method to ``IPagelet`` which was
not specified but used.

* Fixed a infinite recursion bug when a layout template was registered for "*"
but no content template was registered for a pagelet.