[SPARK-4397][Core] Cleanup 'import SparkContext._' in core

This PR cleans up `import SparkContext._` in core for SPARK-4397(#3262) to prove it really works well.

Author: zsxwing <zsxwing@gmail.com>

Closes #3530 from zsxwing/SPARK-4397-cleanup and squashes the following commits:

04e2273 [zsxwing] Cleanup 'import SparkContext._' in core
[SPARK-4686] Link to allowed master URLs is broken

The link points to the old scala programming guide; it should point to the submitting applications page.

This should be backported to 1.1.2 (it's been broken as of 1.0).

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #3542 from kayousterhout/SPARK-4686 and squashes the following commits:

a8fc43b [Kay Ousterhout] [SPARK-4686] Link to allowed master URLs is broken
Indent license header properly for interfaces.scala.

A very small nit update.

Author: Reynold Xin <rxin@databricks.com>

Closes #3552 from rxin/license-header and squashes the following commits:

df8d1a4 [Reynold Xin] Indent license header properly for interfaces.scala.
[SPARK-4536][SQL] Add sqrt and abs to Spark SQL DSL

Spark SQL has embeded sqrt and abs but DSL doesn't support those functions.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3401 from sarutak/dsl-missing-operator and squashes the following commits:

07700cf [Kousuke Saruta] Modified Literal(null, NullType) to Literal(null) in DslQuerySuite
8f366f8 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into dsl-missing-operator
1b88e2e [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into dsl-missing-operator
0396f89 [Kousuke Saruta] Added sqrt and abs to Spark SQL DSL
[SPARK-4663][sql]add finally to avoid resource leak

Author: baishuo <vc_java@hotmail.com>

Closes #3526 from baishuo/master-trycatch and squashes the following commits:

d446e14 [baishuo] correct the code style
b36bf96 [baishuo] correct the code style
ae0e447 [baishuo] add finally to avoid resource leak
[SPARK-4676][SQL] JavaSchemaRDD.schema may throw NullType MatchError if sql has null

val jsc = new org.apache.spark.api.java.JavaSparkContext(sc)
val jhc = new org.apache.spark.sql.hive.api.java.JavaHiveContext(jsc)
val nrdd = jhc.hql("select null from spark_test.for_test")
println(nrdd.schema)
Then the error is thrown as follows:
scala.MatchError: NullType (of class org.apache.spark.sql.catalyst.types.NullType$)
at org.apache.spark.sql.types.util.DataTypeConversions$.asJavaDataType(DataTypeConversions.scala:43)

Author: YanTangZhai <hakeemzhai@tencent.com>
Author: yantangzhai <tyz0303@163.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #3538 from YanTangZhai/MatchNullType and squashes the following commits:

e052dff [yantangzhai] [SPARK-4676] [SQL] JavaSchemaRDD.schema may throw NullType MatchError if sql has null
4b4bb34 [yantangzhai] [SPARK-4676] [SQL] JavaSchemaRDD.schema may throw NullType MatchError if sql has null
896c7b7 [yantangzhai] fix NullType MatchError in JavaSchemaRDD when sql has null
6e643f8 [YanTangZhai] Merge pull request #11 from apache/master
e249846 [YanTangZhai] Merge pull request #10 from apache/master
d26d982 [YanTangZhai] Merge pull request #9 from apache/master
76d4027 [YanTangZhai] Merge pull request #8 from apache/master
03b62b0 [YanTangZhai] Merge pull request #7 from apache/master
8a00106 [YanTangZhai] Merge pull request #6 from apache/master
cbcba66 [YanTangZhai] Merge pull request #3 from apache/master
cdef539 [YanTangZhai] Merge pull request #1 from apache/master
[SPARK-4593][SQL] Return null when denominator is 0

SELECT max(1/0) FROM src
would return a very large number, which is obviously not right.
For hive-0.12, hive would return `Infinity` for 1/0, while for hive-0.13.1, it is `NULL` for 1/0.
I think it is better to keep our behavior with newer Hive version.
This PR ensures that when the divider is 0, the result of expression should be NULL, same with hive-0.13.1

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #3443 from adrian-wang/div and squashes the following commits:

2e98677 [Daoyuan Wang] fix code gen for divide 0
85c28ba [Daoyuan Wang] temp
36236a5 [Daoyuan Wang] add test cases
6f5716f [Daoyuan Wang] fix comments
cee92bd [Daoyuan Wang] avoid evaluation 2 times
22ecd9a [Daoyuan Wang] fix style
cf28c58 [Daoyuan Wang] divide fix
2dfe50f [Daoyuan Wang] return null when divider is 0 of Double type
[SPARK-4670] [SQL] wrong symbol for bitwise not

We should use `~` instead of `-` for bitwise NOT.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #3528 from adrian-wang/symbol and squashes the following commits:

affd4ad [Daoyuan Wang] fix code gen test case
56efb79 [Daoyuan Wang] ensure bitwise NOT over byte and short persist data type
f55fbae [Daoyuan Wang] wrong symbol for bitwise not
[SPARK-4695][SQL] Get result using executeCollect

Using ```executeCollect``` to collect the result, because executeCollect is a custom implementation of collect in spark sql which better than rdd's collect

Author: wangfei <wangfei1@huawei.com>

Closes #3547 from scwf/executeCollect and squashes the following commits:

a5ab68e [wangfei] Revert "adding debug info"
a60d680 [wangfei] fix test failure
0db7ce8 [wangfei] adding debug info
184c594 [wangfei] using executeCollect instead collect
Minor nit style cleanup in GraphX.
[Release] Translate unknown author names automatically
[SPARK-4672][GraphX]Perform checkpoint() on PartitionsRDD to shorten the lineage

The related JIRA is https://issues.apache.org/jira/browse/SPARK-4672

Iterative GraphX applications always have long lineage, while checkpoint() on EdgeRDD and VertexRDD themselves cannot shorten the lineage. In contrast, if we perform checkpoint() on their ParitionsRDD, the long lineage can be cut off. Moreover, the existing operations such as cache() in this code is performed on the PartitionsRDD, so checkpoint() should do the same way. More details and explanation can be found in the JIRA.

Author: JerryLead <JerryLead@163.com>
Author: Lijie Xu <csxulijie@gmail.com>

Closes #3549 from JerryLead/my_graphX_checkpoint and squashes the following commits:

d1aa8d8 [JerryLead] Perform checkpoint() on PartitionsRDD not VertexRDD and EdgeRDD themselves
ff08ed4 [JerryLead] Merge branch 'master' of https://github.com/apache/spark
c0169da [JerryLead] Merge branch 'master' of https://github.com/apache/spark
52799e3 [Lijie Xu] Merge pull request #1 from apache/master
[SPARK-4672][GraphX]Non-transient PartitionsRDDs will lead to StackOverflow error

The related JIRA is https://issues.apache.org/jira/browse/SPARK-4672

In a nutshell, if `val partitionsRDD` in EdgeRDDImpl and VertexRDDImpl are non-transient, the serialization chain can become very long in iterative algorithms and finally lead to the StackOverflow error. More details and explanation can be found in the JIRA.

Author: JerryLead <JerryLead@163.com>
Author: Lijie Xu <csxulijie@gmail.com>

Closes #3544 from JerryLead/my_graphX and squashes the following commits:

628f33c [JerryLead] set PartitionsRDD to be transient in EdgeRDDImpl and VertexRDDImpl
c0169da [JerryLead] Merge branch 'master' of https://github.com/apache/spark
52799e3 [Lijie Xu] Merge pull request #1 from apache/master