Blockdiag

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* Refactor cleanup procedures
* Fix bugs

- Fix get_image_size() closes Image object automatically
- Fix pasting Image object failed on SVG mode
- Fix 61 images.urlopen() got PermissionError on Windows
- Fix 61 images.open() raises exception if ghostscript not found
- Fix 62 Use "sans-serif" to font-family property on SVG output ("sansserif" is invalid)
- Fix 63 AttributeError on get_image_size(); Pillow (<= 2.4.x) does not have Imageclose()

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* Fix bugs

- Fix RuntimeError on unloading plugins

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* Show warnings on loading plugin multiple times
* Unload all plugins on shutdown
* Fix bugs

- Fix caption is wrapped by paragraph node in reST parser

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* Add node event: build_finished
* Take config object to plugins
* Fix bugs

- Fix utils.images.get_image_size() does not close an image descriptor

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* :caption: option of blockdiag directive recognizes inline markups
* Fix bugs

- Fix 58 failed to handle diagram definitions from stdin in py3

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* Show warnings on loading imagedrawers in debug mode
* ImageDrawimage() accepts Image objects
* Fix bugs

- PNG: could not load png imagedrawer if could not access PIL.PILLOW_VERSION