Psd-tools2

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- Fixed parsing of layer groups without explicit OPEN_FOLDER mark;
- Cython extension is rebuilt with Cython 0.18.

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- Descriptor parsing (thanks Oliver Zheng);
- text (as string) is extracted from text layers (thanks Oliver Zheng);
- improved support for optional building of Cython extension.

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- Typo is fixed: ``LayerRecord.cilpping`` should be ``LayerRecord.clipping``.
Thanks Oliver Zheng.

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- Highly experimental: basic layer merging is implemented
(e.g. it is now possible to export layer group to a PIL image);
- ``Layer.visible`` no longer takes group visibility in account;
- ``Layer.visible_global`` is the old ``Layer.visible``;
- ``psd_tools.user_api.combined_bbox`` made public;
- ``Layer.width`` and ``Layer.height`` are removed (use ``layer.bbox.width``
and ``layer.bbox.height`` instead);
- ``pil_support.composite_image_to_PIL`` is renamed to ``pil_support.extract_composite_image`` and
``pil_support.layer_to_PIL`` is renamed to ``pil_support.extract_layer_image``
in order to have the same API for ``pil_support`` and ``pymaging_support``.

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- ``psd.composite_image()`` is renamed to ``psd.as_PIL()``;
- Pymaging support: ``psd.as_pymaging()`` and ``layer.as_pymaging()`` methods.

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- Support for zip and zip-with-prediction compression methods is added;
- support for 16/32bit layers is added;
- optional Cython extension for faster zip-with-prediction decompression;
- other speed improvements.