Upgrade to Assimp 3.1.1

https://github.com/assimp/assimp/releases/tag/v3.1.1

This commit imports assimp 3.1.1, including CHANGES, CREDITS, LICENSE,
README, Readme.md, and code, contrib, include directories. contrib/zlib,
contrib/cppunit-1.12.1/, contrib/cppunit_note.txt were excluded in this
change.
assimp.pri was also updated.
revision.h was generated from revision.h.in.

Change-Id: I7c0597eae9eee7f06a1170968b5fef6fd46b6f0d
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>