update README.md
update upstream mupdf code
revert last commit
Page::renderImage(): return a copyed image

A copyed image can void memory management problems.
update build instruction for MSYS MinGW
remove file infomation
use utf8 encoding for file path when openning the file
modify .travis.yml to test release build
update mupdf source to 1.5
Binding loadDocument with QByteArray