Expose sc.version in PySpark

As discussed here, I think it would be nice if there was a way to programmatically determine what version of Spark you are running.

The potential use cases are not that important, but they include:

1. Branching your code based on what version of Spark is running.
2. Checking your version without having to quit and restart the Spark shell.

Right now in PySpark, I believe the only way to determine your version is by firing up the Spark shell and looking at the startup banner.