Update global asset routing.

Instead of having "assetIMG", "assetCSS", "assetJS" there is now a
generic "globalAsset" which handles routing for /js/, /img/, and /css/
because the routing is so similar.

Also, removed the unnecessary routes.
Add post-specific asset routing.

A post can have images that pertain solely to that post. There is not
point in having these images in the global img asset-dir. This also
allows each post's directory to contain everything that post needs to be
"complete".

This argument extends to img's, js, and future assets.
Update helper.js to create js/css/img directories
Add test post. Includes relative path to CSS file
Fix trailing slash/relative paths issue.

If a post did not have a trailing slash, relative paths were not being followed as expected.

If the following post:
	site.com/posts/some-post
Has a link with a relative path to: css/styles.css

Then the link to the CSS file would become:
	site.com/posts/css/styles.css

This is clearly not what we want. Forcing posts to have a trailing slash eliminates this issue.