Blender Git Loki

API:
- replaced BKE_copy_images with BKE_export_image, now it handles only one image at a time, this is better since for exporters it is
easier to export one image at a time writing new image path to a file
- exposing BKE_export_image in RNA as Image.export, interestingly, RNA allowed me to define a function with PROP_STRING return type
although it doesn't free memory, will fix that in the next commit
- removed bpy.util.copy_images

Unit tests:
- re-wrote a test for BKE_export_image, it's more compact now
- moved unit tests to the creator module to avoid another executable, now running tests with `blender --test`
- as before, unit tests are built only if WITH_BF_UNIT_TEST is non 0