Spk support

I extended jdeb with a new Maven Mojo and a Ant task that supports the creation of packages/archives for Synoloy DiskStations in the spk format. I tried to reuse as much as possible the already existing functions and utilies and also refactored and generalized where necessary.

Please review the changes and let me know, if you need me to change anything. I really would like to see this functionality in jdeb.