Initial work for updating fletch to target python3.
I still haven't gotten this to build all the way through yet, so for now its just a work in progress.
This combines PRs #172 #173 #174 #175 #177 and #186 which each pertain to their own library --- opencv, caffe (which is in there twice), vtk, ffmpeg, and openblas --- and then adds initial support for python3, which is used by multiple libraries.
This branch merges the branches from the other PRs I've recently submitted and then adds initial support for python3 on top. By default only one package (OpenBLAS) has changed versions because it seems that this update never caused any issues in my tests. The other packages still hicup every so often when switching to their new version (some more than others).