node renaming and attrs merging

Added node renaming. See if you like it. 
Storing names in :attrs would require searching through 
the whole graph when referring by name, this is faster. 

Let me know how you want to deal with merging attributes.
In renaming function I have "default" :attrs processor 
(essentially identity) and an option of providing one. 
Maybe when merging :attrs the same should be done.

The question is in what ns it should happen. Attr includes
graph, so merging has to be moved somewhere where
attrs can be required?

Let me know what you think. I'll start implementing 
"graph scalar" for core.matrix this week.