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/contents.py

@@ -17,7 +17,7 @@ from pelican import signals
 from pelican.settings import _DEFAULT_CONFIG
 from pelican.utils import (slugify, truncate_html_words, memoized, strftime,
                            python_2_unicode_compatible, deprecated_attribute,
-                           split_all)
+                           path_to_url)
 
 # Import these so that they're avalaible when you import from pelican.contents.
 from pelican.urlwrappers import (URLWrapper, Author, Category, Tag)  # NOQA
@@ -139,7 +139,7 @@ class Content(object):
         metadata = copy.copy(self.metadata)
         path = self.metadata.get('path', self.get_relative_source_path())
         metadata.update({
-            'path': '/'.join(split_all(path)),
+            'path': path_to_url(path),
             'slug': getattr(self, 'slug', ''),
             'lang': getattr(self, 'lang', 'en'),
             'date': getattr(self, 'date', datetime.now()),