updated comment
fixed ustr helper function
Labels needs unicode support.

str() changed to unicode safe util.ustr() for labels as suggested by Nigel in https://github.com/nigelsmall/py2neo/pull/222.
Merge pull request #223 from johanlundberg/patch-1

Labels needs unicode support.
fixed error raising for Cypher transactions
Merge branch 'master' into release/1.6.2
fixes #231; more test coverage