Pyflakes fixes

Ran pyflakes and fixed several issues:
```
./setup.py:9: redefinition of unused 'setup' from line 7
./beaver/zmq_transport.py:1: 'datetime' imported but unused
./beaver/redis_transport.py:1: 'datetime' imported but unused
./beaver/mqtt_transport.py:1: 'datetime' imported but unused
./beaver/mqtt_transport.py:36: local variable 'timestamp' is assigned to but never used
./beaver/sqs_transport.py:4: 'datetime' imported but unused
./beaver/stdout_transport.py:1: 'datetime' imported but unused
./beaver/rabbitmq_transport.py:1: 'datetime' imported but unused
./beaver/udp_transport.py:1: 'datetime' imported but unused
./beaver/tests/test_zmq_transport.py:2: 'os' imported but unused
./beaver/tests/test_zmq_transport.py:31: local variable 'transport' is assigned to but never used
./beaver/tests/test_transport_config.py:4: 'os' imported but unused
./beaver/tests/test_transport_config.py:10: 'setup_custom_logger' imported but unused
```