Update for Sketch 3

Bohemian Coding removed allSlices and slices methods in Sketch 3 so the plugin was not working anymore. Unfortunately this change is not documented in their documentation.

Using class-dump I have discovered something called exportableLayers that includes slices and all layers that were made exportable.

I used this property and changed code occurences of slices to exportable layers. Because name slice is not relevant anymore.

I also added myself to contributors. I hope you don't mind.