utils: Add path_to_url() and generalize get_relative_path()

The old get_relative_path() implementation assumed os.sep == '/',
which doesn't hold on MS Windows.  The new implementation uses
split_all() for a more general component count.

I added path_to_url(), because the:

  '/'.join(split_all(path))

idiom was showing up in a number of cases, and it's easier to
understand what's going on when that reads:

  path_to_url(path)

This will fix a number of places where I think paths and URLs were
conflated, and should improve MS Windows support.

pelican/utils.py

@@ -304,11 +304,20 @@ def clean_output_dir(path):
 
 def get_relative_path(path):
     """Return the relative path from the given path to the root path."""
-    nslashes = path.count('/')
-    if nslashes == 0:
+    components = split_all(path)
+    if len(components) <= 1:
         return os.curdir
     else:
-        return '/'.join([os.pardir] * nslashes)
+        parents = [os.pardir] * (len(components) - 1)
+        return os.path.join(*parents)
+
+
+def path_to_url(path):
+    """Return the URL corresponding to a given path."""
+    if os.sep == '/':
+        return path
+    else:
+        '/'.join(split_all(path))
 
 
 def truncate_html_words(s, num, end_text='...'):