Bugfix to keep/drop in order to make the preserved nodes
*globally* accessible.
The "globalness" of preserved references depends on the implementation
of require/define, which is not a good thing. This commit fixes that
by getting the global namespace and putting a reference to preserved
nodes there.
Merge pull request #2 from srikumarks/graphnode_keep_drop_fix

Bugfix to keep/drop in order to make the preserved nodes *globally* accessible.