Merge pull request #264 from pryds/master

Lower case strings
lua : dt.modules.* return a function to crate a type object instead of a new object
lua : add dt.debug.type()

this function works exactly like the system type() function, but it will properly report DT specific types
lua : correct a bug that could cause DT to crash in case of lua errors
lua : add a new event, intermediate-export-image

this event is called during the export of an image after the exported image is created but before the storage module has a chance to deal with it
Fix compuation of radius, fix drawing arrow to circle and not center.

Take to opportunity to set radius as constant.
Updated Danish translation.
Danish translation: 'historik' is better than 'historie'.