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/generators.py

@@ -565,7 +565,7 @@ class StaticGenerator(Generator):
                 f_rel = os.path.relpath(f, self.path)
                 # TODO remove this hardcoded 'static' subdirectory
                 dest = os.path.join('static', f_rel)
-                url = '/'.join(pelican.utils.split_all(dest))
+                url = pelican.utils.path_to_url(dest)
                 sc = Static(
                     content=None,
                     metadata={