Blender Git Loki

API:
- freeing strings returned by RNA struct functions in RNA_parameter_list_free

Unit tests:
- check that BKE_export_image actually creates a file. This test is becoming dangerous: it creates and deletes files under /tmp.

Having written this complicated test function I now realize it's much easier to write tests in a scripted language, which gives more
freedom in expressions and need not be compiled.