SPARK-1254. Consolidate, order, and harmonize repository declarations in Maven/SBT builds

This suggestion addresses a few minor suboptimalities with how repositories are handled.

1) Use HTTPS consistently to access repos, instead of HTTP

2) Consolidate repository declarations in the parent POM file, in the case of the Maven build, so that their ordering can be controlled to put the fully optional Cloudera repo at the end, after required repos. (This was prompted by the untimely failure of the Cloudera repo this week, which made the Spark build fail. #2 would have prevented that.)

3) Update SBT build to match Maven build in this regard

4) Update SBT build to not refer to Sonatype snapshot repos. This wasn't in Maven, and a build generally would not refer to external snapshots, but I'm not 100% sure on this one.

Author: Sean Owen <sowen@cloudera.com>

Closes #145 from srowen/SPARK-1254 and squashes the following commits:

42f9bfc [Sean Owen] Use HTTPS for repos; consolidate repos in parent in order to put optional Cloudera repo last; harmonize SBT build repos with Maven; remove snapshot repos from SBT build which weren't in Maven
SPARK-1255: Allow user to pass Serializer object instead of class name for shuffle.

This is more general than simply passing a string name and leaves more room for performance optimizations.

Note that this is technically an API breaking change in the following two ways:
1. The shuffle serializer specification in ShuffleDependency now require an object instead of a String (of the class name), but I suspect nobody else in this world has used this API other than me in GraphX and Shark.
2. Serializer's in Spark from now on are required to be serializable.

Author: Reynold Xin <rxin@apache.org>

Closes #149 from rxin/serializer and squashes the following commits:

5acaccd [Reynold Xin] Properly call serializer's constructors.
2a8d75a [Reynold Xin] Added more documentation for the serializer option in ShuffleDependency.
7420185 [Reynold Xin] Allow user to pass Serializer object instead of class name for shuffle.
SPARK-1240: handle the case of empty RDD when takeSample

https://spark-project.atlassian.net/browse/SPARK-1240

It seems that the current implementation does not handle the empty RDD case when run takeSample

In this patch, before calling sample() inside takeSample API, I add a checker for this case and returns an empty Array when it's a empty RDD; also in sample(), I add a checker for the invalid fraction value

In the test case, I also add several lines for this case

Author: CodingCat <zhunansjtu@gmail.com>

Closes #135 from CodingCat/SPARK-1240 and squashes the following commits:

fef57d4 [CodingCat] fix the same problem in PySpark
36db06b [CodingCat] create new test cases for takeSample from an empty red
810948d [CodingCat] further fix
a40e8fb [CodingCat] replace if with require
ad483fd [CodingCat] handle the case with empty RDD when take sample
SPARK-1244: Throw exception if map output status exceeds frame size

This is a very small change on top of @andrewor14's patch in #147.

Author: Patrick Wendell <pwendell@gmail.com>
Author: Andrew Or <andrewor14@gmail.com>

Closes #152 from pwendell/akka-frame and squashes the following commits:

e5fb3ff [Patrick Wendell] Reversing test order
393af4c [Patrick Wendell] Small improvement suggested by Andrew Or
8045103 [Patrick Wendell] Breaking out into two tests
2b4e085 [Patrick Wendell] Consolidate Executor use of akka frame size
c9b6109 [Andrew Or] Simplify test + make access to akka frame size more modular
281d7c9 [Andrew Or] Throw exception on spark.akka.frameSize exceeded + Unit tests
[Spark-1261] add instructions for running python examples to doc overview page

Author: Diana Carroll <dcarroll@cloudera.com>

Closes #162 from dianacarroll/SPARK-1261 and squashes the following commits:

14ac602 [Diana Carroll] typo in python example text
5121e3e [Diana Carroll] Add explanation of how to run Python examples to main doc overview page
Spark 1246 add min max to stat counter

Here's the addition of min and max to statscounter.py and min and max methods to rdd.py.

Author: Dan McClary <dan.mcclary@gmail.com>

Closes #144 from dwmclary/SPARK-1246-add-min-max-to-stat-counter and squashes the following commits:

fd3fd4b [Dan McClary] fixed  error, updated test
82cde0e [Dan McClary] flipped incorrectly assigned inf values in StatCounter
5d96799 [Dan McClary] added max and min to StatCounter repr for pyspark
21dd366 [Dan McClary] added max and min to StatCounter output, updated doc
1a97558 [Dan McClary] added max and min to StatCounter output, updated doc
a5c13b0 [Dan McClary] Added min and max to Scala and Java RDD, added min and max to StatCounter
ed67136 [Dan McClary] broke min/max out into separate transaction, added to rdd.py
1e7056d [Dan McClary] added underscore to getBucket
37a7dea [Dan McClary] cleaned up boundaries for histogram -- uses real min/max when buckets are derived
29981f2 [Dan McClary] fixed indentation on doctest comment
eaf89d9 [Dan McClary] added correct doctest for histogram
4916016 [Dan McClary] added histogram method, added max and min to statscounter
Revert "SPARK-1236 - Upgrade Jetty to 9.1.3.v20140225."

This reverts commit ca4bf8c572c2f70b484830f1db414b5073744ab6.

Jetty 9 requires JDK7 which is probably not a dependency we want to bump right now. Before Spark 1.0 we should consider upgrading to Jetty 8. However, in the mean time to ease some pain let's revert this. Sorry for not catching this during the initial review. cc/ @rxin

Author: Patrick Wendell <pwendell@gmail.com>

Closes #167 from pwendell/jetty-revert and squashes the following commits:

811b1c5 [Patrick Wendell] Revert "SPARK-1236 - Upgrade Jetty to 9.1.3.v20140225."
Merge branch 'master' of https://github.com/apache/spark into SPARK-1256