Update slug format for post type

Posts no longer need to specify a full url for the resulting html.
Rather, the prefix portion of the url will be inferred from the
post's location in the source filesystem. The post slug now simply
represents a url friendly version of the title.

Additionally, updates all sample posts to use the new slug format.

gatsby/on-create-node.js

@@ -8,10 +8,11 @@ const onCreateNode = ({ node, actions, getNode }) => {
 
   if (node.internal.type === 'MarkdownRemark') {
     if (typeof node.frontmatter.slug !== 'undefined') {
+      const dirname = getNode(node.parent).relativeDirectory;
       createNodeField({
         node,
         name: 'slug',
-        value: node.frontmatter.slug
+        value: `/${dirname}/${node.frontmatter.slug}`
       });
     } else {
       const value = createFilePath({ node, getNode });