Fixed bug in join_polygon_objects()

When objects being nested in the same hierarchy were passed to join_polygon_objects(), their global position was not retained. This was fixed by transferring the global position to the clone.
Fixed bug in current_state_to_object()

When the object passed current_state_to_object() was already bound in a hierarchy, the function failed. This was fixed, by creating a clone of the object.
Merge branch 'master' into development
closes issue #2.

If the symbolcache could not be written because of missing permissions, the new field of the Resource class "missing_permissions" can be used to determine this.
Important: The interface of `c4dtools.resource.load()` has changed. It returns a tuple of three elements instead of two now.
Updated README.md