add overlay Contours at map and configuration

This change added a new overlay layer into the map. This new overlay layer show hill contours. Similar to the option 'Show Hillshadow by Default' a new configuration options is added called 'Show Contours by Default'.

Term of use for 'Contours' can you found here http://www.openmapsurfer.uni-hd.de/contact.html - In my opinion is okay to use this service.