[SPARK-1617] and [SPARK-1618] Improvements to streaming ui and bug fix to socket receiver

1617: These changes expose the receiver state (active or inactive) and last error in the UI
1618: If the socket receiver cannot connect in the first attempt, it should try to restart after a delay. That was broken, as the thread that restarts (hence, stops) the receiver waited on Thread.join on itself!

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

Closes #540 from tdas/streaming-ui-fix and squashes the following commits:

e469434 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into streaming-ui-fix
dbddf75 [Tathagata Das] Style fix.
66df1a5 [Tathagata Das] Merge remote-tracking branch 'apache/master' into streaming-ui-fix
ad98bc9 [Tathagata Das] Refactored streaming listener to use ReceiverInfo.
d7f849c [Tathagata Das] Revert "Moved BatchInfo from streaming.scheduler to streaming.ui"
5c80919 [Tathagata Das] Moved BatchInfo from streaming.scheduler to streaming.ui
da244f6 [Tathagata Das] Fixed socket receiver as well as made receiver state and error visible in the streamign UI.
Generalize pattern for planning hash joins.

This will be helpful for [SPARK-1495](https://issues.apache.org/jira/browse/SPARK-1495) and other cases where we want to have custom hash join implementations but don't want to repeat the logic for finding the join keys.

Author: Michael Armbrust <michael@databricks.com>

Closes #418 from marmbrus/hashFilter and squashes the following commits:

d5cc79b [Michael Armbrust] Address @rxin 's comments.
366b6d9 [Michael Armbrust] style fixes
14560eb [Michael Armbrust] Generalize pattern for planning hash joins.
f4809c1 [Michael Armbrust] Move common functions to PredicateHelper.
[Typo] In the maven docs: chd -> cdh

Author: Andrew Or <andrewor14@gmail.com>

Closes #548 from andrewor14/doc-typo and squashes the following commits:

3eaf4c4 [Andrew Or] chd -> cdh
Fix [SPARK-1078]: Remove the Unnecessary lift-json dependency

Remove the Unnecessary lift-json dependency from pom.xml

Author: Sandeep <sandeep@techaddict.me>

Closes #536 from techaddict/FIX-SPARK-1078 and squashes the following commits:

bd0fd1d [Sandeep] Fix [SPARK-1078]: Replace lift-json with json4s-jackson. Remove the Unnecessary lift-json dependency from pom.xml
SPARK-1242 Add aggregate to python rdd

Author: Holden Karau <holden@pigscanfly.ca>

Closes #139 from holdenk/add_aggregate_to_python_api and squashes the following commits:

0f39ae3 [Holden Karau] Merge in master
4879c75 [Holden Karau] CR feedback, fix issue with empty RDDs in aggregate
70b4724 [Holden Karau] Style fixes from code review
96b047b [Holden Karau] Add aggregate to python rdd
Call correct stop().

Oopsie in #504.

Author: Aaron Davidson <aaron@databricks.com>

Closes #527 from aarondav/stop and squashes the following commits:

8d1446a [Aaron Davidson] Call correct stop().