Merge branch 'master' of https://github.com/rk700/PyMuPDF

Bug fixing PDF string escapes (pymupdf#153)

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Updated PDFjoiner.py

All Changes for MuPDF 1.9a

Fixing bugs

* fixed invalid json format when images occur
* immunize against invalid invocation of doc.select() method (invalid
page number range, invalid document type)

Update performance documentation

Performance details on basic operation and text extraction.

Delete crashes_2.txt

support MuPDF 1.2