Better feature detection for assimp version

The unified headers were introduced in Assimp 2.0.1150, so checking for Assimp 3.0.0 is not quite the best solution.

See https://github.com/assimp/assimp/commit/6fa251c2f2e7a142bb861227dce0c26362927fbc
Updated Assimp defines to be more flexible

This commit is a follow-up to 85b20197671e142044e471df603debd0faf08baf

Why was export.h removed from assimp < 3.0.0?
Invert Assimp version detect logic for greater accuracy
Fix export API detection (for assimp < 2.0.885)

It looks like this API was added in Assimp 2.0.885:
https://github.com/assimp/assimp/commit/ae23c03bd9a0b5f1227dc0042fd98f7206c770a8
Use VERSION_LESS instead of STRLESS

The version comparison routines were added in cmake 2.8.0
updating changelog
1.11.0
undo incorrect version
do not use visual and collision group and remove visual and collision if there is no vertices
fix problem of root coordinates
fix debug message