Cattrs

- Distribution fix.

- Removed the undocumented `Converter.unstruct_strat` property setter.
- Removed the ability to set the `Converter.structure_attrs` instance field.
- Some micro-optimizations were applied; a `structure(unstructure(obj))` roundtrip
is now up to 2 times faster.

- Packaging fixes.
([17](https://github.com/python-attrs/cattrs/pull/17))

- structure/unstructure now supports using functions as well as classes for deciding the appropriate function.
- added `Converter.register_structure_hook_func`, to register a function instead of a class for determining handler func.
- added `Converter.register_unstructure_hook_func`, to register a function instead of a class for determining handler func.
- vendored typing is no longer needed, nor provided.
- Attributes with default values can now be structured if they are missing in the input.
([15](https://github.com/python-attrs/cattrs/pull/15))
- `Optional` attributes can no longer be structured if they are missing in the input.
- `cattr.typed` removed since the functionality is now present in _attrs_ itself.
Replace instances of `cattr.typed(type)` with `attr.ib(type=type)`.

- `Converter.loads` is now `Converter.structure`, and `Converter.dumps` is now `Converter.unstructure`.
- Python 2.7 is supported.
- Moved `cattr.typing` to `cattr.vendor.typing` to support different vendored versions of typing.py for Python 2 and Python 3.
- Type metadata can be added to _attrs_ classes using `cattr.typed`.

- Python 3.4 is no longer supported.
- Introduced `cattr.typing` for use with Python versions 3.5.2 and 3.6.0.
- Minor changes to work with newer versions of `typing`.
- Bare Optionals are not supported any more (use `Optional[Any]`).
- Attempting to load unrecognized classes will result in a ValueError, and a helpful message to register a loads hook.
- Loading _attrs_ classes is now documented.
- The global converter is now documented.
- `cattr.loads_attrs_fromtuple` and `cattr.loads_attrs_fromdict` are now exposed.