New 0.17.3 release

This release contains the following changes :
+ avoid SWT Exceptions raised on diagram changes
+ introduce new zoom levels
+ correct drawing errors on diagrams link icon
+ store URLs instead of file path in diagrams links
+ embed new Violet 0.17a (that contains changes written previously and some other technical changes)
+ improve diagram change detection
+ improve drag'n drop by unselecting all items on the current diagram
+ tested on Linux Knoppix 4.0.2 and Sun Java Virtual Machine 1.5.04
+ add Cay S. Horstmann in the plugin authors list
+ packaged as a zip archive since severals users told me that they had problems with the jar file in previous releases