ReadBatchWith bug fix and new MaxWaitTime option (segmentio#482)

This fixes a bug where ReadBatchWith could incorrectly return `io.EOF`
in certain `errShortRead` scenarios.  Since `io.EOF` means the batch
was handled correctly, this could cause consumers to continue to use
a connection that is in a bad state.

This also adds a MaxWait option that can be used to improve the
way that the conn deadline and the fetch request interact.  Instead
of trying to calculate a correct timeout and change the max wait
time, this allows the caller to explicitly configure both the wait
and the deadline independently of one another.

0.4: fix short writes (segmentio#479)

Merge remote-tracking branch 'origin/0.4' into 0.4-writer

Merge pull request segmentio#462 from segmentio/fix-retries-3

Revert segmentio#382

Merge branch '0.4' of ssh://github.com/segmentio/kafka-go into 0.4

Ensure Dialer.DialPartition honors configured timeout and deadline (s…

…egmentio#425)

add kafka.ApiVersion.Format (segmentio#395)

* add kafka.ApiVersion.Format

* set explicit constant values intead of using iota

Updated constant for `FirstOffset` with correct type (segmentio#368)

Closes segmentio#367

ReaderConfig.RetentionTime usage (segmentio#341)

Using RetentionTime from ReaderConfig on create NewConsumerGroup

don't use io.StringWriter (segmentio#337)