Add tests to cover LogStash::Event date formats
Make our wrapped errors include the original exception class and text.
use ISODateTimeFormat for parsing @timestamp strings

- new timestamp tests in spec/event.rb now pass with this change.
- Also removed a comment on LogStash::Time that was no longer accurate.
Add specific examples from LOGSTASH-1738 and LOGSTASH-1732
Merge pull request #915 from jordansissel/backport-pull-913-date-parse-fixes

Backport pull 913 date parse fixes