Assimp can now be built as a subproject in CMake based projects

Signed-off-by: Filip Wasil <filip.wasil@gmail.com>
Merge pull request #299 from filipwasil/builder_fix

Assimp can now be built as a subproject in CMake based projects
ObjExporter: cleanup code. Fix bug writing normal: normals would not be transformed by world transform.
Merge branch 'master' of github.com:assimp/assimp
FBXConverter: avoid creating an empty metadata slot (UDP3DSMAX was erased from list _after_ its size was taken)/
SceneCombiner: implement proper copying of meta data. This entire module will need to be revamped in future as it is highly likely to cause further regressions as assimp data structures are modified.
Fix memory corruption in the aiGetExportFormatDescription() API. Clarify Exporter doc.
Update CHANGES for 3.1.1 (forgot before)