Replaces Imgwriter node with a scifio-based reimplantation.

The new Imgwriter node sports the following features:
- based on scifio
- more options in the node settings
    - custom file name prefix
    - create nonexistent folders  
- various bugfixes



This node requires the still unreleased scifio 0.16 and scifio-ome-xml 0.9.4, to write ome-xml files.