Add `raise_send_errors` flag to the base `Client`.
This indicates whether the client will handle or raise any exceptions encountered while sending to Sentry.
* The default value (False) matches the current behaviour in which the client handles the error.
* Setting this argument to True allows for the calling application to handle the exception and
  thus determine its own retry and backoff strategies.
If a SentryHandler's client is set with `raise_send_errors = True`, do not handle the errors in `SentryHandler.emit`.
Merge branch 'raise_send_errors' of https://github.com/richg/raven-python into richg-raise_send_errors

Conflicts:
	raven/base.py
	raven/contrib/django/client.py
	raven/handlers/logbook.py
	raven/handlers/logging.py
	tests/handlers/logging/tests.py
Merge branch 'master' of github.com:getsentry/raven-python