Add support for throwable to Status

[ubschmidt2]

Mentioned in https://github.com/census-instrumentation/opencensus-java/pull/779#pullrequestreview-74281349.

Also, it would be nice to let the exporter send the stacktrace, too, at least in case of stackdriver trace (https://cloud.google.com/trace/docs/reference/v2/rpc/google.devtools.cloudtrace.v2#google.devtools.cloudtrace.v2.Span), not sure about other backends.

[codefromthecrypt]

Definitely would be cool. We need to watch carefully where and which parsing cost (of throwable) is added