[SPARK-2492][Streaming] kafkaReceiver minor changes to align with Kafka 0.8

Update the KafkaReceiver's behavior when auto.offset.reset is set.

In Kafka 0.8, `auto.offset.reset` is a hint for out-range offset to seek to the beginning or end of the partition. While in the previous code `auto.offset.reset` is a enforcement to seek to the beginning or end immediately, this is different from Kafka 0.8 defined behavior.

Also deleting extesting ZK metadata in Receiver when multiple consumers are launched will introduce issue as mentioned in [SPARK-2383](https://issues.apache.org/jira/browse/SPARK-2383).

So Here we change to offer user to API to explicitly reset offset before create Kafka stream, while in the meantime keep the same behavior as Kafka 0.8 for parameter `auto.offset.reset`.

@tdas, would you please review this PR? Thanks a lot.

Author: jerryshao <saisai.shao@intel.com>

Closes #1420 from jerryshao/kafka-fix and squashes the following commits:

d6ae94d [jerryshao] Address the comment to remove the resetOffset() function
de3a4c8 [jerryshao] Fix compile error
4a1c3f9 [jerryshao] Doc changes
b2c1430 [jerryshao] Move offset reset to a helper function to let user explicitly delete ZK metadata by calling this API
fac8fd6 [jerryshao] Changes to align with Kafka 0.8