- add metadata support to aiScene. Some doc cleanup. Add const versions of aiScene::aiNode.
- STEP: fix bug that caused back references contained in nested aggregates to be not tracked.
- IFC: add support for reading IfcProperties as node metadata.
- Ifc: prevent infinite recursion due to cyclic back references which the new STEP backref tracker might produce.
- add metadata.h to cmake.
- update docs.
- Ifc: support reading IfcComplexProperty
- Ifc: limit nesting of IfcComplexProperty's.
Merge pull request #21 from assimp/assimp-metadata

Assimp metadata implementation. This adds aiNode::mMetaData, the <assimp/metadata.h> header containing the aiMetaData structure. Also adds support for Ifc reading metadata to test out the new feature.