Adding a layer to its own list of guide layers breaks snapping for that layer

[rawbeans]

If you do the following:

 layer.snapediting.addGuideLayer(layer);

layer will no longer snap to any other guide layer because layer is always the closest guide layer to itself.

Obviously you can check if a layer is equal to the layer you are adding it to, but it might be a good idea to check for this within the library. It is never desirable to have a layer within its guide layer array because it breaks snapping, so might as well prevent it from ever happening.

[haoliangyu]

This has been fixed by #30, where I add a function (source) to detect whether a guide layer was the editing layer itself (then skip it). I don't remove a layer from its guide group because it make it possible to use the same feature group with the L.EditToolbar.SnapEdit.