Adding tessellation support for new Blender 2.6 polygons

- Added BlenderBMesh.cpp/h which contains a class to convert a BMesh to an old style tri/quad mesh
- Added BlenderTessellator.cpp/h which contains a class to tessellate the poly loops contained within a BMesh
Update CMakeLists
BlenderTesselator: disable GLU tesselation code by default.
BlenderTesselator: Fix OffsetOf() macro
Merge pull request #187 from assimp/blender-26-poly

Blender 2.6 poly format patch, by @kittencataclysm. This enables reading the new mesh format in Blender 2.6 and thus fixes a whole bunch of reports related to recent Blender versions.