SPARK-2143 [WEB UI] Add Spark version to UI footer

This PR adds the Spark version number to the UI footer; this is how it looks:

![screen shot 2014-11-21 at 22 58 40](https://cloud.githubusercontent.com/assets/822522/5157738/f4822094-7316-11e4-98f1-333a535fdcfa.png)

Author: Sean Owen <sowen@cloudera.com>

Closes #3410 from srowen/SPARK-2143 and squashes the following commits:

e9b3a7a [Sean Owen] Add Spark version to footer
[HOTFIX] Fix build break in 1a2508b73f6d46a0faf7740b85a5c216c925c25a

org.apache.spark.SPARK_VERSION is new in 1.2; in earlier versions,
we have to use SparkContext.SPARK_VERSION.