[WebUI] Fix collapse of WebUI layout

When we decrease the width of browsers, the header of WebUI wraps and collapses like as following image.

![2015-01-11 19 49 37](https://cloud.githubusercontent.com/assets/4736016/5698887/b0b9aeee-99cd-11e4-9020-08f3f0014de0.png)

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3995 from sarutak/fixed-collapse-webui-layout and squashes the following commits:

3e60b5b [Kousuke Saruta] Modified line-height property in webui.css
7bfb5fb [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into fixed-collapse-webui-layout
5d83e18 [Kousuke Saruta] Fixed collapse of WebUI layout

core/src/main/resources/org/apache/spark/ui/static/webui.css

@@ -19,6 +19,7 @@
   height: 50px;
   font-size: 15px;
   margin-bottom: 15px;
+  min-width: 1200px
 }
 
 .navbar .navbar-inner {
@@ -39,12 +40,12 @@
 
 .navbar .nav > li a {
   height: 30px;
-  line-height: 30px;
+  line-height: 2;
 }
 
 .navbar-text {
   height: 50px;
-  line-height: 50px;
+  line-height: 3.3;
 }
 
 table.sortable thead {
@@ -170,7 +171,7 @@ span.additional-metric-title {
 }
 
 .version {
-  line-height: 30px;
+  line-height: 2.5;
   vertical-align: bottom;
   font-size: 12px;
   padding: 0;