pelican/utils: Add split_all() and rework static URL generation

I think the conversion from native paths to URLs is best put off until
we are actually trying to generate the URL.  The old handling
(introduced in 2692586, Fixes #645 - Making cross-content linking
windows compatible, 2012-12-19) converted the path at StaticContent
initialization, which left you with a bogus StaticContent.src.

Once we drop the 'static' subdirectory, we will be able to drop the
`dest` and `url` parts from the StaticGenerator.generate_context()
handling, which will leave things looking a good deal cleaner than
they do now.

pelican/utils.py

@@ -480,3 +480,24 @@ def mkdir_p(path):
     except OSError as e:
         if e.errno != errno.EEXIST or not os.path.isdir(path):
             raise
+
+
+def split_all(path):
+    """Split a path into a list of components
+
+    While os.path.split() splits a single component off the back of
+    `path`, this function splits all components:
+
+    >>> split_all(os.path.join('a', 'b', 'c'))
+    ['a', 'b', 'c']
+    """
+    components = []
+    while path:
+        head,tail = os.path.split(path)
+        if tail:
+            components.insert(0, tail)
+        elif head == path:
+            components.insert(0, head)
+            break
+        path = head
+    return components