[SPARK-20360][PYTHON] reprs for interpreters

## What changes were proposed in this pull request?

Establishes a very minimal `_repr_html_` for PySpark's `SparkContext`.

## How was this patch tested?

nteract:

![screen shot 2017-04-17 at 3 41 29 pm](https://cloud.githubusercontent.com/assets/836375/25107701/d57090ba-2385-11e7-8147-74bc2c50a41b.png)

Jupyter:

![screen shot 2017-04-17 at 3 53 19 pm](https://cloud.githubusercontent.com/assets/836375/25107725/05bf1fe8-2386-11e7-93e1-07a20c917dde.png)

Hydrogen:

![screen shot 2017-04-17 at 3 49 55 pm](https://cloud.githubusercontent.com/assets/836375/25107664/a75e1ddc-2385-11e7-8477-258661833007.png)

Author: Kyle Kelley <rgbkrk@gmail.com>

Closes #17662 from rgbkrk/repr.

python/pyspark/context.py

@@ -240,6 +240,32 @@ class SparkContext(object):
         if isinstance(threading.current_thread(), threading._MainThread):
             signal.signal(signal.SIGINT, signal_handler)
 
+    def __repr__(self):
+        return "<SparkContext master={master} appName={appName}>".format(
+            master=self.master,
+            appName=self.appName,
+        )
+
+    def _repr_html_(self):
+        return """
+        <div>
+            <p><b>SparkContext</b></p>
+
+            <p><a href="{sc.uiWebUrl}">Spark UI</a></p>
+
+            <dl>
+              <dt>Version</dt>
+                <dd><code>v{sc.version}</code></dd>
+              <dt>Master</dt>
+                <dd><code>{sc.master}</code></dd>
+              <dt>AppName</dt>
+                <dd><code>{sc.appName}</code></dd>
+            </dl>
+        </div>
+        """.format(
+            sc=self
+        )
+
     def _initialize_context(self, jconf):
         """
         Initialize SparkContext in function to allow subclass specific initialization

python/pyspark/sql/session.py

@@ -221,6 +221,17 @@ class SparkSession(object):
                 or SparkSession._instantiatedSession._sc._jsc is None:
             SparkSession._instantiatedSession = self
 
+    def _repr_html_(self):
+        return """
+            <div>
+                <p><b>SparkSession - {catalogImplementation}</b></p>
+                {sc_HTML}
+            </div>
+        """.format(
+            catalogImplementation=self.conf.get("spark.sql.catalogImplementation"),
+            sc_HTML=self.sparkContext._repr_html_()
+        )
+
     @since(2.0)
     def newSession(self):
         """