Remove hardcoded Spark version string to use SBT defined version instead

This PR attempts to remove hardcoded references to the Spark version which must be updated for each release.

This PR uses the version defined in the main sbt definition file (project/SparkBuild.scala - ``SPARK_VERSION``). When the sbt compile/package/assembly is triggered, this string is used as the version in JAR manifest.

We then use ``getClass.getPackage.getImplementationVersion`` to refer to this string.

As a demonstration of potential use cases, the master ui page is modified to display the Spark version.