Handle copying assets from assets-only directories

Fixes #39

Previously when Gatsby was copying assets in the post-build step, it
assumed that all assets were in the same directory as an html page as
page paths can be written programatically, to ensure assets are copied
to the right directory, they too need to follow however the page path
is modified.

This created a bug however when assets were placed in a directory
without a page as then the code couldn't find a page associated with the
assets.

This commit checks now if page is returned undefined and if it is,
copies the asset to /public to a directory with the same name e.g.
/pages/images/foo.jpg is copied to /public/images/foo.jpg.

lib/utils/post-build.coffee

@@ -33,7 +33,16 @@ module.exports = (program, cb) ->
           if page.file.name.slice(0,1) isnt '_'
             parsePath(page.requirePath).dirname is oldPath
 
-        newPath = parsePath(page.path).dirname + parsed.basename
+        if page?
+          newPath = parsePath(page.path).dirname + parsed.basename
+
+        # If a page wasn't found, this probably means the asset is in
+        # a folder without a page e.g. an images directory. In this case,
+        # there is no path rewriting so just copy to a directory with
+        # the same name.
+        else
+          relativePath = path.relative(directory + "/pages", parsed.dirname)
+          newPath = "#{relativePath}/#{parsed.basename}"
 
       newPath = directory + "/public/" + newPath
       fs.copy(file, newPath, (err) ->