adds v14-pre: fix go.mod & go install

ref #134

create v13.1.1

updates shopify/sarama to v1.31.0

tweaks docs.

consume: distinguishes nil and empty slice. #67

updates travis go version to 1.9 🎉

group: sync result collection. #47

adds new method to consumer mock.

uses sarama's offset manager to mark offsets.

adds leading 0 to versions and missing config.