[tools] terrain various fixes

Thanks to Charles L. for the feedback.

- add strength option
- colorspace_settings new in 2.64
- grid `radius` is new in 2.68, was `size` in 2.67a:

http://www.blender.org/documentation/blender_python_api_2_67a_release/bpy.ops.mesh.html#bpy.ops.mesh.primitive_grid_add

http://www.blender.org/documentation/blender_python_api_2_68_release/bpy.ops.mesh.html#bpy.ops.mesh.primitive_grid_add