SPARK-1556. jets3t dep doesn't update properly with newer Hadoop versions

See related discussion at https://github.com/apache/spark/pull/468

This PR may still overstep what you have in mind, but let me put it on the table to start. Besides fixing the issue, it has one substantive change, and that is to manage Hadoop-specific things only in Hadoop-related profiles. This does _not_ remove `yarn.version`.

- Moves the YARN and Hadoop profiles together in pom.xml. Sorry that this makes the diff a little hard to grok but the changes are only as follows.
- Removes `hadoop.major.version`
- Introduce `hadoop-2.2` and `hadoop-2.3` profiles to control Hadoop-specific changes:
  - like the protobuf version issue - this was only 'solved' now by enabling YARN for 2.2+, which is really an orthogonal issue
  - like the jets3t version issue now
- Hadoop profiles set an appropriate default `hadoop.version`, that can be overridden
- _(YARN profiles in the parent now only exist to add the sub-module)_
- Fixes the jets3t dependency issue
 - and makes it a runtime dependency
 - and centralizes config of this guy in the parent pom
- Updates build docs
- Updates SBT build too
  - and fixes a regex problem along the way

Author: Sean Owen <sowen@cloudera.com>

Closes #629 from srowen/SPARK-1556 and squashes the following commits:

c3fa967 [Sean Owen] Fix hadoop-2.4 profile typo in doc
a2105fd [Sean Owen] Add hadoop-2.4 profile and don't set hadoop.version in profiles
274f4f9 [Sean Owen] Make jets3t a runtime dependency, and bring its exclusion up into parent config
bbed826 [Sean Owen] Use jets3t 0.9.0 for Hadoop 2.3+ (and correct similar regex issue in SBT build)
f21f356 [Sean Owen] Build changes to set up for jets3t fix
SPARK-1721: Reset the thread classLoader in the Mesos Executor

This is because Mesos calls it with a different environment or something, the result is that the Spark jar is missing and it can't load classes.

This fixes http://apache-spark-user-list.1001560.n3.nabble.com/java-lang-ClassNotFoundException-spark-on-mesos-td3510.html

I have no idea whether this is the right fix, I can only confirm that it fixes the issue for us.

The `registered` method is called from mesos (https://github.com/apache/mesos/blob/765ff9bc2ac5a12d4362f8235b572a37d646390a/src/java/jni/org_apache_mesos_MesosExecutorDriver.cpp)

I am unsure which commit caused this regression

Author: Bouke van der Bijl <boukevanderbijl@gmail.com>

Closes #620 from bouk/mesos-classloader-fix and squashes the following commits:

c13eae0 [Bouke van der Bijl] Use getContextOrSparkClassLoader in SparkEnv and CompressionCodec