title: multiple SparkContexts error in tutorial
body: <p>I am attempting to run the very basic Spark+Python pyspark tutorial -- see <a href="http://spark.apache.org/docs/0.9.0/quick-start.html">http://spark.apache.org/docs/0.9.0/quick-start.html</a></p>
  
  <p>When I attempt to initialize a new SparkContext,</p>
  
  <pre><code>from pyspark import SparkContext
  sc = SparkContext("local[4]", "test")
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>ValueError: Cannot run multiple SparkContexts at once
  </code></pre>
  
  <p>I'm wondering if my previous attempts at running example code loaded something into memory that didn't clear out.  Is there a way to list current SparkContexts already in memory and/or clear them out so the sample code will run?</p>