GBE-533: added post-operation for json-based printing
GBE-533: post implementation in PrintController + type info added to
json mapper + protect against nulls in json
GBE-533: replaced generic IsInfo mixin by specific mixins because of
enum problem (gwt-jackson does not put typeinfo in enums) + default to
pdf when filename has no extension
Merge pull request #66 from geomajas/GBE-533-stateless-print

Gbe 533 stateless print