Psd-tools2

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- minor bugfix.

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- change _psd pointer in PSDImage;
- add version property;
- support fill effects in composer.

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- change tagged block/image resource singleton accessor in user API;
- add documentation on iterator order;
- fix export setting 1 big key config;
- fix computer info big key config.

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- add channel size checking in topil;
- add mlst metadata decoding;
- fix key collision issue in descriptor;
- performance improvement for packbit encoding/decoding;
- drop cython dependency in travis config;
- implement thumbnail, is_group, and parent methods in PSDImage.

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- major API changes;
- package name changed to `psd_tools2`;
- completely rewritten decoding subpackage `psd_tools2.psd`;
- improved composer functionality;
- file write support;
- drop cython compression module and makes the package pure-python;
- drop pymaging support.

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- composer alpha blending fix;
- documentation fix.