Pharo3.0: trying to merge gitfiletree and add traits support

Hi @dalehenrich,

Can you help me merge this? I got a bit lost between upstream and my fork, but at the same time this is a completely updated and tested merge of gitfiletree for pharo3.0 (Travis may not agree with me, still).

(There a MonticelloFileTree-Core change I picked on my way to a maze of merge conflicts in git :()