Change back to string format and only stdout with evalResult

[zhouzhuojie]

Use plain text as console stdout, and log data logging with fmt.Println

[zhouzhuojie]

example

INFO[0006] started handling request                      method=POST remote="127.0.0.1:58344" request=/api/v1/evaluation
{"FlagEvalResult":{"evalContext":{"enableDebug":true,"entityContext":{"state":"NY"},"entityID":"127","entityType":"user","flagID":2},"evalDebugLog":{"segmentDebugLogs":[{"msg":"matched all constraints. rollout yes. {BucketNum:807 DistributionArray:{VariantIDs:[4] PercentsAccumulated:[1000]} VariantID:4 RolloutPercent:88}","segmentID":2}]},"flagID":2,"segmentID":2,"timestamp":"2017-10-28T00:25:59Z","variantAttachment":{"color":"simple-blue"},"variantID":4,"variantKey":"blue"}}