Included changes

• New webserver that Joy Gao has been working on.
  
  
• New timezone feature
  
• Remove the legacy import style that's been deprecated since at least 1.8
  
• Move API out of experimental
• Remove Pickle support
• Update Airflow documentation on airflow.apache.org
• Refactor sensor packages
  https://github.com/apache/incubator-airflow/pull/2875