fixed __str__ output for node 0 and rel 0
fixed __str__ output for node 0 and rel 0

nicolewhite/py2neo,pombreda/py2neo,pombreda/py2neo,technige/py2neo,fpieper/py2neo,pombreda/py2neo,fpieper/py2neo,fpieper/py2neo,technige/py2neo,nigelsmall/py2neo,nicolewhite/py2neo,nigelsmall/py2neo,technige/py2neo,nicolewhite/py2neo