db_pool: customizable connection cleanup function; Thanks to Avery Fay

https://github.com/eventlet/eventlet/pull/64

Also:
- PEP8
- except Exception
- .put() must not catch SystemExit
Python 3 compat: Use correct _fileobject
Python 3 compat: Use bytes when reading/writing
Python3 compat: InstanceType/__new__; Thanks to Jakub Stasiak
greenio, tpool: python3 compatibility

Also:
- PEP-8
- check both EAGAIN/EWOULDBLOCK
- use system implementation of GreenPipe.readinto()
Python 3 compat: Derive exceptions from Exception
Python 3 compat: Remove some unicode and str refs
Python 3 compat: Use bytes where appropriate