Fix a bug due to Q.all() running concurrent operations in parallel.

ElementWrapper.setProperties() must issue an async operation per (key,value)
in the properties. TinkerGraph does not support multiple concurrent
operations, so we must force the operations to be performed sequentially.
Merge pull request #4 from jimlloyd/feature/fix-set-properties-bug

Fix a bug due to Q.all() running concurrent operations in parallel.