Added LZ4 to compression codec in configuration page.

Author: Reynold Xin <rxin@apache.org>

Closes #1417 from rxin/lz4 and squashes the following commits:

472f6a1 [Reynold Xin] Set the proper default.
9cf0b2f [Reynold Xin] Added LZ4 to compression codec in configuration page.

docs/configuration.md

@@ -336,13 +336,12 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.io.compression.codec</code></td>
-  <td>org.apache.spark.io.<br />LZFCompressionCodec</td>
+  <td>org.apache.spark.io.<br />SnappyCompressionCodec</td>
   <td>
     The codec used to compress internal data such as RDD partitions and shuffle outputs.
-    By default, Spark provides two codecs: <code>org.apache.spark.io.LZFCompressionCodec</code>
-    and <code>org.apache.spark.io.SnappyCompressionCodec</code>. Of these two choices,
-    Snappy offers faster compression and decompression, while LZF offers a better compression
-    ratio.
+    By default, Spark provides three codecs:  <code>org.apache.spark.io.LZ4CompressionCodec</code>,
+    <code>org.apache.spark.io.LZFCompressionCodec</code>,
+    and <code>org.apache.spark.io.SnappyCompressionCodec</code>.
   </td>
 </tr>
 <tr>