Address issue #550: exception chaining.
Address issue #550: exception chaining.

Enhance raven.events.Exception to report the exception chain
when __suppress_context__ is supported.

johansteffner/raven-python,johansteffner/raven-python,johansteffner/raven-python,getsentry/raven-python,getsentry/raven-python,getsentry/raven-python