[REEF-14]: A logo for REEF

* Added logo to website

JIRA: [REEF-14](https://issues.apache.org/jira/browse/REEF-14)

Pull Request:
  This closes #321

website/src/site/resources/css/site.css

@@ -0,0 +1,21 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+.pull-left {
+  margin: 10px
+}

website/src/site/site.xml

@@ -45,7 +45,9 @@ under the License.
     </custom>
 
   <bannerLeft>
-    <src>REEFLogo.png</src>
+    <name>Apache REEF</name>
+    <src>ApacheREEF_logo_no_margin_small.png</src>
+    <alt>Apache REEF</alt>
     <href></href>
   </bannerLeft>
   <bannerRight>