Variety of fixes for compactor bugs

- Improved the example script with a ping/pong demo
- Added support for LIBPROCESS_* environment variables
- Better exception handling when socket creation fails
- Change outgoing sockets to nonblocking
- Dropped support legacy response codes (mesos < 0.19.0)
- Fixed tornado keep-alive handling
- Ensure new HTTP requests include the content length header
- Fixed installation through pip (with a custom build base directory)