Updated terminology: circle -> marker.
Extracted Marker class.
Renamed circle field to marker.
Renamed fits viewer states to refer to markers instead of circles.
Added cross to Marker using Line2D's.  When rendered it is at a large 
constant offset from where it should be though.
Removed test data directory gitignore.
Fixed coordinate offset for crosshair on source marker.
Decreased linewidth of crosshair.
Remove crosshair from axes when removing Marker from axes.
Merge pull request #184 from drusk/crosshair

Closes #183.