Next GeoMason pre-release committed to repository

Highlights:

• Heavily refactored importers:
• ArcInfoASCGridImporter supports compressed streams (see Sleuth demo); should also have some performance improvement
• other importers now use URL instead of String; demos updated accordingly
• made importers static classes, so no longer necessary to instantiate them
• ShapeFileImporter now has more convenience member functions
• Compressed repository large demo data files, no 83Mb instead of 1.4Gb