[SQL] Improve column pruning in the optimizer.

Author: Michael Armbrust <michael@databricks.com>

Closes #378 from marmbrus/columnPruning and squashes the following commits:

779da56 [Michael Armbrust] More consistent naming.
1a4e9ea [Michael Armbrust] More comments.
2f4e7b9 [Michael Armbrust] Improve column pruning in the optimizer.
SPARK-1202 - Add a "cancel" button in the UI for stages

Author: Sundeep Narravula <sundeepn@superduel.local>
Author: Sundeep Narravula <sundeepn@dhcpx-204-110.corp.yahoo.com>

Closes #246 from sundeepn/uikilljob and squashes the following commits:

5fdd0e2 [Sundeep Narravula] Fix test string
f6fdff1 [Sundeep Narravula] Format fix; reduced line size to less than 100 chars
d1daeb9 [Sundeep Narravula] Incorporating review comments.
8d97923 [Sundeep Narravula] Ability to kill jobs thru the UI. This behavior can be turned on be settings the following variable: spark.ui.killEnabled=true (default=false) Adding DAGScheduler event StageCancelled and corresponding handlers. Added cancellation reason to handlers.
Set spark.executor.uri from environment variable (needed by Mesos)

The Mesos backend uses this property when setting up a slave process.  It is similarly set in the Scala repl (org.apache.spark.repl.SparkILoop), but I couldn't find any analogous for pyspark.

Author: Ivan Wick <ivanwick+github@gmail.com>

This patch had conflicts when merged, resolved by
Committer: Matei Zaharia <matei@databricks.com>

Closes #311 from ivanwick/master and squashes the following commits:

da0c3e4 [Ivan Wick] Set spark.executor.uri from environment variable (needed by Mesos)
Add Spark v0.9.1 to ec2 launch script and use it as the default

Mainly ported from branch-0.9.

Author: Harvey Feng <hyfeng224@gmail.com>

Closes #385 from harveyfeng/0.9.1-ec2 and squashes the following commits:

769ac2f [Harvey Feng] Add Spark v0.9.1 to ec2 launch script and use it as the default
SPARK-1202: Improvements to task killing in the UI.

1. Adds a separate endpoint for the killing logic that is outside of a page.
2. Narrows the scope of the killingEnabled tracking.
3. Some style improvements.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #386 from pwendell/kill-link and squashes the following commits:

8efe02b [Patrick Wendell] Improvements to task killing in the UI.