Fix failure to fetch CSS.

Bug: CSS can't be fetched after installing the gem.
Expectation: CSS should be loaded properly from the header.html file.
Replication: Just install the gem on a jekyll site.
Cause: The base url is not properly configured.
Solution: Change {{ site.baseurl }} to "/". poole/hyde#213

_includes/head.html

@@ -15,10 +15,10 @@
   </title>
 
   <!-- CSS -->
-  <link rel="stylesheet" href="{{ site.baseurl }}public/css/poole.css">
-  <link rel="stylesheet" href="{{ site.baseurl }}public/css/syntax.css">
-  <link rel="stylesheet" href="{{ site.baseurl }}public/css/hyde.css">
-  <link rel="stylesheet" href="{{ site.baseurl }}public/css/hyde-plus.css">
+  <link rel="stylesheet" href="/public/css/poole.css">
+  <link rel="stylesheet" href="/public/css/syntax.css">
+  <link rel="stylesheet" href="/public/css/hyde.css">
+  <link rel="stylesheet" href="/public/css/hyde-plus.css">
   <link rel="stylesheet" href="http://fonts.googleapis.com/css?family=PT+Sans:400,400italic,700|Abril+Fatface">
   <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">