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* Preliminary support for programmatic access to error details (Issue 5).
There are certainly some corner cases that don't do the right thing yet, but
this works mostly.
In order to make this happen (and also to clean things up a bit), a number
of deprecations are necessary:
* ``stop_on_error`` is deprecated in ``Validator.__init__``. Use
``Validator.iter_errors()`` instead.
* ``number_types`` and ``string_types`` are deprecated there as well.
Use ``types={"number" : ..., "string" : ...}`` instead.
* ``meta_validate`` is also deprecated, and instead is now accepted as
an argument to ``validate``, ``iter_errors`` and ``is_valid``.
* A bugfix or two