Updated README.txt
Track references and normal elements separately in Cell

When building the dependency tree, we currently need to go through *all* elements of a cell,
even though we are mostly only interested in the references.

This commit saves the references and elements in separate lists, resulting in a potentially
huge speedup during save.
Merge pull request #8 from mabl/references_optimize

Track references and normal elements separately in Cell
Merge branch 'master' of https://github.com/hohlraum/gdsCAD
Merge branch 'master' into pull_7b

Conflicts:
	gdsCAD/core.py
Save always performs minimal uniquification.