Set descriptive front page title

The title of the homepage is used in Google search results.

layouts/header.html

@@ -32,7 +32,7 @@
     <% if @item[:title] %>
     <title><%= @item[:title] %> | Prometheus</title>
     <% else %>
-    <title>Prometheus</title>
+    <title>Prometheus - Service monitoring and time series database</title>
     <% end %>
 
     <!-- Bootstrap core CSS -->