0.15 release
Bump version to 0.16.0.dev
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
tests: env_test: ProcessBase -> run_python

Extracted inline code to permanent files
Python3 compatibility of test files