Make Jekyll Rouge syntax highlighting work.

Added assets/css/syntax.css that controls syntax highlighting colors.
This file is simply the monokai Pygments theme with the background color
replaced with the existing background color for <pre>. The colors are a
decent starting point, but they certainly aren't perfect.

Added some content to assets/css/screen.edited.css to style the syntax
highlighted code correctly. There were several display issues to fix,
especially when "lineno" is enabled.

Added code to _layouts/default.html to include the syntax.css file.

Adjusted the style test to include some syntax highlighting with line
numbers.

Author: vexingcodes

_layouts/default.html

@@ -14,6 +14,7 @@
     <!-- Styles'n'Scripts -->
     <link rel="stylesheet" type="text/css" href="{{ site.baseurl }}assets/built/screen.css" />
     <link rel="stylesheet" type="text/css" href="{{ site.baseurl }}assets/built/screen.edited.css" />
+    <link rel="stylesheet" type="text/css" href="{{ site.baseurl }}assets/built/syntax.css" />
     <!-- highlight.js -->
     <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/default.min.css">
     <style>.hljs { background: none; }</style>

_posts/2012-09-01-a-full-and-comprehensive-style-test.html

@@ -149,7 +149,7 @@ <h3 id="code">Code</h3>
 {% endhighlight %}
 
 
-{% highlight javascript %}
+{% highlight javascript linenos %}
 // javascript code sample
 $.ajax({
   type: 'POST',