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`.
Use lstrip to get rid of leading slashes, because in some cases the paths have already been replaced.
Merge pull request #443 from Pr0Ger/master

Removed unused assignment
Ensure that ThreadedHTTPTransport sends all pending messages at exit

A race condition in the ThreadedHTTPTransport exit hook meant that it was
possible for some queued messages to not be sent.

Rearrange the shutdown hook a bit to ensure that this cannot happen.
Improve behavior when API returns valid errors
Some brief docs on transports
Suggest sync DSN in test example
Expand usage
Expand usage
Correct option ref to verify_ssl
Enable SSL verification by default
Initial notes for 4.2
Document transport options
Example of transport options
Improve readability on transport options
Add missing import
Update required Sentry version
Explicitly catch AttributeError
Add rq documentation