Python docstring update for sql.py.

Mostly related to the following two rules in PEP8 and PEP257:
- Line length < 72 chars.
- First line should be a concise description of the function/class.

Author: Reynold Xin <rxin@apache.org>

Closes #869 from rxin/docstring-schemardd and squashes the following commits:

7cf0cbc [Reynold Xin] Updated sql.py for pep8 docstring.
0a4aef9 [Reynold Xin] Merge branch 'master' into docstring-schemardd
6678937 [Reynold Xin] Python docstring update for sql.py.
Fix PEP8 violations in Python mllib.

Author: Reynold Xin <rxin@apache.org>

Closes #871 from rxin/mllib-pep8 and squashes the following commits:

848416f [Reynold Xin] Fixed a typo in the previous cleanup (c -> sc).
a8db4cd [Reynold Xin] Fix PEP8 violations in Python mllib.
SPARK-1903 Document Spark's network connections

https://issues.apache.org/jira/browse/SPARK-1903

Author: Andrew Ash <andrew@andrewash.com>

Closes #856 from ash211/SPARK-1903 and squashes the following commits:

6e7782a [Andrew Ash] Add the technology used on each port
1d9b5d3 [Andrew Ash] Document port for history server
56193ee [Andrew Ash] spark.ui.port becomes worker.ui.port and master.ui.port
a774c07 [Andrew Ash] Wording in network section
90e8237 [Andrew Ash] Use real :toc instead of the hand-written one
edaa337 [Andrew Ash] Master -> Standalone Cluster Master
57e8869 [Andrew Ash] Port -> Default Port
3d4d289 [Andrew Ash] Title to title case
c7d42d9 [Andrew Ash] [WIP] SPARK-1903 Add initial port listing for documentation
a416ae9 [Andrew Ash] Word wrap to 100 lines
[SQL] Minor: Introduce SchemaRDD#aggregate() for simple aggregations

```scala
rdd.aggregate(Sum('val))
```
is just shorthand for

```scala
rdd.groupBy()(Sum('val))
```

but seems be more natural than doing a groupBy with no grouping expressions when you really just want an aggregation over all rows.

Did not add a JavaSchemaRDD or Python API, as these seem to be lacking several other methods like groupBy() already -- leaving that cleanup for future patches.

Author: Aaron Davidson <aaron@databricks.com>

Closes #874 from aarondav/schemardd and squashes the following commits:

e9e68ee [Aaron Davidson] Add comment
db6afe2 [Aaron Davidson] Introduce SchemaRDD#aggregate() for simple aggregations
HOTFIX: Add no-arg SparkContext constructor in Java

Self explanatory.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #878 from pwendell/java-constructor and squashes the following commits:

2cc1605 [Patrick Wendell] HOTFIX: Add no-arg SparkContext constructor in Java