Fix Jekyll Rouge syntax highlighting.

[vexingcodes]

Make Jekyll Rouge syntax highlighting work. This is an attempt at fixing #21.

• 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 tags. 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. I'm terrible at CSS, so this is probably not the minimum amount required to do the job, but it appears to function.
• Added code to _layouts/default.html to include the syntax.css file.
• Adjusted the style test to include some syntax highlighting with line numbers.

The following is a screenshot of what the changes look like on the updated style test page. The screenshot doesn't show this, but if the comment is too long for the current width of the box then a horizontal scrollbar will appear, so no wrapping occurs.

[biomadeira]

@vexingcodes thanks a lot for your help ;)

F:)