Add topic and partition to ConsumerEvent struct
Update docs and sendError
Merge pull request #91 from Shopify/export_topic_and_partition_in_consumer_event_struct

Add topic and partition to ConsumerEvent struct
Move ConsumerConfig validation to separate method that is exported.
Add docs for ConsumerConfig#Validate.
Do the same for ClientConfig and ProducerConfig
Add NewConsumerConfig()
Add NewProducerConfig
Add NewClientConfig