Packing Slip HTML Filter
Add filter to override packing slip html function controller_packing_slip.
Filter will make it possible for plugins to add to packing slip, for example adding
pre-printed shipping lables.

Also, fixed a couple of whitespace issues in function controller_packing_slip
changed name of filter to wpsc_packing_packing_slip_path
Merge pull request #859 from JeffPyeBrook/packing-slip-filter

Packing Slip filter