[WP8] Optimize .XAP size

This prevent the .XML documentation files (MonoGame.Framework.xml,
SharpDX.xml, etc) to get copied into the .XAP file. Those are text files
and compress well in the xap but take significant space once the app is
deployed.
http://stackoverflow.com/questions/2011434/preventing-referenced-assembly-pdb-and-xml-files-copied-to-output


I created two projects, Game1 (old template) & Game2 (new template),
build with ARM/Release  and deploy to the phone.

In the /Bin directory:
Game1_Release_ARM.xap 1163 KB
Game2_Release_ARM.xap  572 KB

On the phone (Settings/Phone Storage/Apps)
Game1_Release_ARM.xap 14,15 MB
Game2_Release_ARM.xap  7,20 MB