[SPARK-6807] [SparkR] Merge recent SparkR-pkg changes

This PR pulls in recent changes in SparkR-pkg, including

cartesian, intersection, sampleByKey, subtract, subtractByKey, except, and some API for StructType and StructField.

Author: cafreeman <cfreeman@alteryx.com>
Author: Davies Liu <davies@databricks.com>
Author: Zongheng Yang <zongheng.y@gmail.com>
Author: Shivaram Venkataraman <shivaram.venkataraman@gmail.com>
Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>
Author: Sun Rui <rui.sun@intel.com>

Closes #5436 from davies/R3 and squashes the following commits:

c2b09be [Davies Liu] SQLTypes -> schema
a5a02f2 [Davies Liu] Merge branch 'master' of github.com:apache/spark into R3
168b7fe [Davies Liu] sort generics
b1fe460 [Davies Liu] fix conflict in README.md
e74c04e [Davies Liu] fix schema.R
4f5ac09 [Davies Liu] Merge branch 'master' of github.com:apache/spark into R5
41f8184 [Davies Liu] rm man
ae78312 [Davies Liu] Merge pull request #237 from sun-rui/SPARKR-154_3
1bdcb63 [Zongheng Yang] Updates to README.md.
5a553e7 [cafreeman] Use object attribute instead of argument
71372d9 [cafreeman] Update docs and examples
8526d2e71 [cafreeman] Remove `tojson` functions
6ef5f2d [cafreeman] Fix spacing
7741d66 [cafreeman] Rename the SQL DataType function
141efd8 [Shivaram Venkataraman] Merge pull request #245 from hqzizania/upstream
9387402 [Davies Liu] fix style
40199eb [Shivaram Venkataraman] Move except into sorted position
07d0dbc [Sun Rui] [SPARKR-244] Fix test failure after integration of subtract() and subtractByKey() for RDD.
7e8caa3 [Shivaram Venkataraman] Merge pull request #246 from hlin09/fixCombineByKey
ed66c81 [cafreeman] Update `subtract` to work with `generics.R`
f3ba785 [cafreeman] Fixed duplicate export
275deb4 [cafreeman] Update `NAMESPACE` and tests
1a3b63d [cafreeman] new version of `CreateDF`
836c4bf [cafreeman] Update `createDataFrame` and `toDF`
be5d5c1 [cafreeman] refactor schema functions
40338a4 [Zongheng Yang] Merge pull request #244 from sun-rui/SPARKR-154_5
20b97a6 [Zongheng Yang] Merge pull request #234 from hqzizania/assist
ba54e34 [Shivaram Venkataraman] Merge pull request #238 from sun-rui/SPARKR-154_4
c9497a3 [Shivaram Venkataraman] Merge pull request #208 from lythesia/master
b317aa7 [Zongheng Yang] Merge pull request #243 from hqzizania/master
136a07e [Zongheng Yang] Merge pull request #242 from hqzizania/stats
cd66603 [cafreeman] new line at EOF
8b76e81 [Shivaram Venkataraman] Merge pull request #233 from redbaron/fail-early-on-missing-dep
7dd81b7 [cafreeman] Documentation
0e2a94f [cafreeman] Define functions for schema and fields