[SPARK-7944] [SPARK-8013] Remove most of the Spark REPL fork for Scala 2.11

This PR removes most of the code in the Spark REPL for Scala 2.11 and leaves just a couple of overridden methods in `SparkILoop` in order to:

- change welcome message
- restrict available commands (like `:power`)
- initialize Spark context

The two codebases have diverged and it's extremely hard to backport fixes from the upstream REPL. This somewhat radical step is absolutely necessary in order to fix other REPL tickets (like SPARK-8013 - Hive Thrift server for 2.11). BTW, the Scala REPL has fixed the serialization-unfriendly wrappers thanks to ScrapCodes's work in [#4522](https://github.com/scala/scala/pull/4522)

All tests pass and I tried the `spark-shell` on our Mesos cluster with some simple jobs (including with additional jars), everything looked good.

As soon as Scala 2.11.7 is out we need to upgrade and get a shaded `jline` dependency, clearing the way for SPARK-8013.

/cc pwendell

Author: Iulian Dragos <jaguarul@gmail.com>

Closes #6903 from dragos/issue/no-spark-repl-fork and squashes the following commits:

c596c6f [Iulian Dragos] Merge branch 'master' into issue/no-spark-repl-fork
2b1a305 [Iulian Dragos] Removed spaces around multiple imports.
0ce67a6 [Iulian Dragos] Remove -verbose flag for java compiler (added by mistake in an earlier commit).
10edaf9 [Iulian Dragos] Keep the jline dependency only in the 2.10 build.
529293b [Iulian Dragos] Add back Spark REPL files to rat-excludes, since they are part of the 2.10 real.
d85370d [Iulian Dragos] Remove jline dependency from the Spark REPL.
b541930 [Iulian Dragos] Merge branch 'master' into issue/no-spark-repl-fork
2b15962 [Iulian Dragos] Change jline dependency and bump Scala version.
b300183 [Iulian Dragos] Rename package and add license on top of the file, remove files from rat-excludes and removed `-Yrepl-sync` per reviewer’s request.
9d46d85 [Iulian Dragos] Fix SPARK-7944.
abcc7cb [Iulian Dragos] Remove the REPL forked code.