Blender: map texture influence to aiTextureType

Previously assimp mapped all blender image textures as aiTextureType_DIFFUSE.  This change interprets the "mapto" property which corresponds to the Blender "Influence" in the properties editor to corresponding aiTextureType. Otherwise the texture type is aiTextureType_UNKNOWN.

Notes:
* Blender's Normal influence with "Normal Map" unchecked maps to aiTextureType_HEIGHT.
* Blender's Normal influence with "Normal Map" checked maps to aiTextureType_NORMAL.
* Normal influence scale maps to AI_MATKEY_BUMPSCALING.

Wasn't sure about the formatting, please let me know, as I can clean it up. This change will break backwards compatibility to code that expected all textures to be mapped to aiTextureType_DIFFUSE.