[SPARK-6128][Streaming][Documentation] Updates to Spark Streaming Programming Guide

Updates to the documentation are as follows:

- Added information on Kafka Direct API and Kafka Python API
- Added joins to the main streaming guide
- Improved details on the fault-tolerance semantics

Generated docs located here
http://people.apache.org/~tdas/spark-1.3.0-temp-docs/streaming-programming-guide.html#fault-tolerance-semantics

More things to add:
- Configuration for Kafka receive rate
- May be add concurrentJobs

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4956 from tdas/streaming-guide-update-1.3 and squashes the following commits:

819408c [Tathagata Das] Minor fixes.
debe484 [Tathagata Das] Added DataFrames and MLlib
380cf8d [Tathagata Das] Fix link
04167a6 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into streaming-guide-update-1.3
0b77486 [Tathagata Das] Updates based on Josh's comments.
86c4c2a [Tathagata Das] Updated streaming guides
82de92a [Tathagata Das] Add Kafka to Python api docs