Import threadweaver

This will be used as one option for the internal tasking API. It will
be possible to also replace this with other options such as Intel
Thread Building Blocks or some future home grown tasking implementation
as needed.

Change-Id: Ic6c6c0d8858eb3adae9fe48133241dec0597c2c8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>