[SPARK-2601] [PySpark] Fix Py4J error when transforming pickleFiles

Similar to SPARK-1034, the problem was that Py4J didn’t cope well with the fake ClassTags used in the Java API.  It doesn’t look like there’s any reason why PythonRDD needs to take a ClassTag, since it just ignores the type of the previous RDD, so I removed the type parameter and we no longer pass ClassTags from Python.

Author: Josh Rosen <joshrosen@apache.org>

Closes #1605 from JoshRosen/spark-2601 and squashes the following commits:

b68e118 [Josh Rosen] Fix Py4J error when transforming pickleFiles [SPARK-2601]
SPARK-2680: Lower spark.shuffle.memoryFraction to 0.2 by default

Author: Matei Zaharia <matei@databricks.com>

Closes #1593 from mateiz/spark-2680 and squashes the following commits:

3c949c4 [Matei Zaharia] Lower spark.shuffle.memoryFraction to 0.2 by default
[SPARK-2361][MLLIB] Use broadcast instead of serializing data directly into task closure

We saw task serialization problems with large feature dimension, which could be avoid if we don't serialize data directly into task but use broadcast variables. This PR uses broadcast in both training and prediction and adds tests to make sure the task size is small.

Author: Xiangrui Meng <meng@databricks.com>

Closes #1427 from mengxr/broadcast-new and squashes the following commits:

b9a1228 [Xiangrui Meng] style update
b97c184 [Xiangrui Meng] minimal change to LBFGS
9ebadcc [Xiangrui Meng] add task size test to RowMatrix
9427bf0 [Xiangrui Meng] add task size tests to linear methods
e0a5cf2 [Xiangrui Meng] add task size test to GD
28a8411 [Xiangrui Meng] add test for NaiveBayes
380778c [Xiangrui Meng] update KMeans test
bccab92 [Xiangrui Meng] add task size test to LBFGS
02103ba [Xiangrui Meng] remove print
e73d68e [Xiangrui Meng] update tests for k-means
174cb15 [Xiangrui Meng] use local-cluster for test with a small akka.frameSize
1928a5a [Xiangrui Meng] add test for KMeans task size
e00c2da [Xiangrui Meng] use broadcast in GD, KMeans
010d076 [Xiangrui Meng] modify NaiveBayesModel and GLM to use broadcast
[SPARK-2679] [MLLib] Ser/De for Double

Added a set of serializer/deserializer for Double in _common.py and PythonMLLibAPI in MLLib.

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1581 from dorx/doubleSerDe and squashes the following commits:

86a85b3 [Doris Xin] Merge branch 'master' into doubleSerDe
2bfe7a4 [Doris Xin] Removed magic byte
ad4d0d9 [Doris Xin] removed a space in unit
a9020bc [Doris Xin] units passed
7dad9af [Doris Xin] WIP
SPARK-2684: Update ExternalAppendOnlyMap to take an iterator as input

This will decrease object allocation from the "update" closure used in map.changeValue.

Author: Matei Zaharia <matei@databricks.com>

Closes #1607 from mateiz/spark-2684 and squashes the following commits:

b7d89e6 [Matei Zaharia] Add insertAll for Iterables too, and fix some code style
561fc97 [Matei Zaharia] Update ExternalAppendOnlyMap to take an iterator as input
[SPARK-2705][CORE] Fixed stage description in stage info page

Stage description should be a `String`, but was changed to an `Option[String]` by mistake:

![stage-desc-small](https://cloud.githubusercontent.com/assets/230655/3655611/f6d0b0f6-117b-11e4-83ed-71000dcd5009.png)

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1524 from liancheng/fix-stage-desc and squashes the following commits:

3c69327 [Cheng Lian] Fixed stage description object type in Web UI stage table