Add export scene feature

With reference to the issue here https://github.com/OpenGeoscience/vtkMap/issues/25
I have added the export scene feature.
The exported image has time stamp as its file name.
The export scene directory can be set similar to the Tile Cache directory (default to ~/.vtkMap/export)
I have added a KeyPress Callback in main for testing the export scene.
@aashish24 Should I remove that callback from main?