Taskjuggler: Add valid project attributes
* ox-taskjuggler.el (org-taskjuggler--build-project): Insert all
valid attributes from project.
* ox-taskjuggler.el (org-taskjuggler--build-attributes): Add project
as possible ITEM to docstring.
* ox-taskjuggler.el (org-taskjuggler-valid-project-attributes): New
defcustom.
As reported by Nick Garber, project attributes like 'timingresolution'
could not be changed via Org-mode properties. This patch makes the
exporter treat project-attributes just like it treats task-, resource-
and report-attributes.