TimeConverter used to use the SimpleDateFormat
The last one is [not thread safe](http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html#synchronization).
So, this commit returns a new SimpleDateFormat instance for every request.
Merge pull request #617 from cucumber/time-converter-simple-date-format-and-thread-safety

TimeConverter used to use the SimpleDateFormat