Proper resize cursors for simple shapes' resize markers in edit mode

When resizing circles or rectangles, the mouse cursor is set to `pointer`, while CSS3 provides better solution - directed arrows known from windows resizing. This change replaces `leaflet-edit-resize` css class with two new: `leaflet-edit-resize-nesw` and `leaflet-edit-resize-nwse` that set the cursor accordingly. Those classes are assigned to the resize markers through Edit.SimpleShape options. 

I haven't changed the name of `resizeIcon` property to `neswResizeIcon` for backward compatibility, but it might make sense to clean it up at future major release.