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