SPARK-1053.  Don't require SPARK_YARN_APP_JAR

It looks this just requires taking out the checks.

I verified that, with the patch, I was able to run spark-shell through yarn without setting the environment variable.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #553 from sryza/sandy-spark-1053 and squashes the following commits:

b037676 [Sandy Ryza] SPARK-1053.  Don't require SPARK_YARN_APP_JAR
SPARK-1078:  Replace lift-json with json4s-jackson.

The aim of the Json4s project is to provide a common API for
Scala JSON libraries.  It is Apache-licensed, easier for
downstream distributions to package, and mostly API-compatible
with lift-json.  Furthermore, the Jackson-backed implementation
parses faster than lift-json on all but the smallest inputs.

Author: William Benton <willb@redhat.com>

Closes #582 from willb/json4s and squashes the following commits:

7ca62c4 [William Benton] Replace lift-json with json4s-jackson.
SPARK-1135: fix broken anchors in docs

A recent PR that added Java vs Scala tabs for streaming also
inadvertently added some bad code to a document.ready handler, breaking
our other handler that manages scrolling to anchors correctly with the
floating top bar. As a result the section title ended up always being
hidden below the top bar. This removes the unnecessary JavaScript code.

Author: Matei Zaharia <matei@databricks.com>

Closes #3 from mateiz/doc-links and squashes the following commits:

e2a3488 [Matei Zaharia] SPARK-1135: fix broken anchors in docs
SPARK-1115: Catch depickling errors

This surroungs the complete worker code in a try/except block so we catch any error that arrives. An example would be the depickling failing for some reason

@JoshRosen

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

Closes #644 from bouk/catch-depickling-errors and squashes the following commits:

f0f67cc [Bouke van der Bijl] Lol indentation
0e4d504 [Bouke van der Bijl] Surround the complete python worker with the try block
Removed reference to incubation in README.md.

Author: Reynold Xin <rxin@apache.org>

Closes #1 from rxin/readme and squashes the following commits:

b3a77cd [Reynold Xin] Removed reference to incubation in README.md.
Merge branch 'master' of github.com:witgo/spark