Error handling & documentation

• this release brings bigger changes to error handling:
  • it cleans up a little middleware stack including smart-notifier errors
  • it introduces two types of errors - unhandled error which is handled the same as till now, and transient error which won't be pushed to sentry, but only instrumented in datadog
  • it deprecates dedicated smartNotifierErrorMiddleware
  • smartNotifierHandler in case of error behaves like notifHandler and pass the error down the middleware stack
• added timeout option to Hull.Connector constructor to control the timeout value
• upgrades raven library
• add support for batch handlers for accounts
• adds users_segments and accounts_segments to Context Object
• deprecation Renamed userHandlerOptions to options in notifyHandler
• flow types fixes