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.

* Blender's Normal influence with "Normal Map" unchecked maps to
  aiTextureType_HEIGHT.
* Blender's Normal influence with "Normal Map" checked maps to
  aiTextureType_NORMAL.
* Influence scale is placed into AI_MATKEY_BUMPSCALING.
Merge pull request #146 from calvindhsu/blender-texture-type

Blender: map texture influence to aiTextureType