[SPARK-5469] restructure pyspark.sql into multiple files

All the DataTypes moved into pyspark.sql.types

The changes can be tracked by `--find-copies-harder -M25`
```
davieslocalhost:~/work/spark/python$ git diff --find-copies-harder -M25 --numstat master..
2       5       python/docs/pyspark.ml.rst
0       3       python/docs/pyspark.mllib.rst
10      2       python/docs/pyspark.sql.rst
1       1       python/pyspark/mllib/linalg.py
21      14      python/pyspark/{mllib => sql}/__init__.py
14      2108    python/pyspark/{sql.py => sql/context.py}
10      1772    python/pyspark/{sql.py => sql/dataframe.py}
7       6       python/pyspark/{sql_tests.py => sql/tests.py}
8       1465    python/pyspark/{sql.py => sql/types.py}
4       2       python/run-tests
1       1       sql/core/src/main/scala/org/apache/spark/sql/test/ExamplePointUDT.scala
```

Also `git blame -C -C python/pyspark/sql/context.py` to track the history.

Author: Davies Liu <[email protected]>

Closes apache#4479 from davies/sql and squashes the following commits:

1b5f0a5 [Davies Liu] Merge branch 'master' of github.com:apache/spark into sql
2b2b983 [Davies Liu] restructure pyspark.sql

Author: Davies Liu

python/docs/pyspark.ml.rst

@@ -1,11 +1,8 @@
 pyspark.ml package
 =====================
 
-Submodules
-----------
-
-pyspark.ml module
------------------
+Module Context
+--------------
 
 .. automodule:: pyspark.ml
     :members:

python/docs/pyspark.mllib.rst

@@ -1,9 +1,6 @@
 pyspark.mllib package
 =====================
 
-Submodules
-----------
-
 pyspark.mllib.classification module
 -----------------------------------
 

python/docs/pyspark.sql.rst

@@ -1,10 +1,18 @@
 pyspark.sql module
 ==================
 
-Module contents
----------------
+Module Context
+--------------
 
 .. automodule:: pyspark.sql
     :members:
     :undoc-members:
     :show-inheritance:
+
+
+pyspark.sql.types module
+------------------------
+.. automodule:: pyspark.sql.types
+    :members:
+    :undoc-members:
+    :show-inheritance:

python/pyspark/mllib/linalg.py

@@ -29,7 +29,7 @@
 
 import numpy as np
 
-from pyspark.sql import UserDefinedType, StructField, StructType, ArrayType, DoubleType, \
+from pyspark.sql.types import UserDefinedType, StructField, StructType, ArrayType, DoubleType, \
     IntegerType, ByteType