Fixes #337.
Fixes #337.

Adds undo/redo for node removal

JKereliuk/phenotips,DeanWay/phenotips,teyden/phenotips,itaiGershtansky/phenotips,phenotips/phenotips,teyden/phenotips,itaiGershtansky/phenotips,alexhenrie/phenotips,veronikaslc/phenotips,DeanWay/phenotips,teyden/phenotips,phenotips/phenotips,teyden/phenotips,alexhenrie/phenotips,itaiGershtansky/phenotips,danielpgross/phenotips,mjshepherd/phenotips,JKereliuk/phenotips,JKereliuk/phenotips,teyden/phenotips,DeanWay/phenotips,tmarathe/phenotips,alexhenrie/phenotips,danielpgross/phenotips,veronikaslc/phenotips,tmarathe/phenotips,phenotips/phenotips,phenotips/phenotips,veronikaslc/phenotips,mjshepherd/phenotips,danielpgross/phenotips,tmarathe/phenotips,phenotips/phenotips,mjshepherd/phenotips