Array properties support for graphml export/import

I added the support for array properties to the graphml-import/export commands.
I also slightly modified the graphml-export command so that it does not export node/edge properties with empty values. 

Please note that the diff stats for the java files I modified do not reflect the real changes, which are smaller than indicated. 
This is due to the fact that I copied the new files over the old ones instead of modifying them directly (unfortunately, at the beginning I didn't not think of pulling requests).
Anyway, I marked the changes I made with my nickname (gquercini).