Pyramid-basemodel

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Fix Python3 support (requires 3.3 for the unicode literal character).

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Allow concrete subclasses of ``BaseModelContainer`` to be provided in the
``tree.BaseContentRoot.mapping`` (as well as interfaces).

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Added a `pyramid_basemodel.blob.Blob` model class to store large binary files.

Fixed up `.slug` module doctests. Fix `install_requires` list in `setup.py`.

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Base the ``BaseMixin.class_name`` on a singularised version of the plural
class name. This may seem a bit arse about face, but allows us to use the
tablename to split the word, e.g.:

.. code-block::

>>> class OperatingScale(Base, BaseMixin):
... __tablename__ 'operating_scales'
...

>>> OperatingScale.class_name
'Operating Scale'

Just ignore all this if you don't plan on using the `class_name`, `class_slug` and
`plural_class_name` properties.

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Provide `BaseMixin.class_slug` and base it and the `BaseMixin.plural_class_name`
on the `cls.__tablename__` instead of the `cls.__name__`.

All still manually overrideable by providing the corresponding property with a
single underscore, e.g.: `cls._plural_class_name`.

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Provide `BaseMixin.class_name` and `BaseMixin.plural_class_name`.