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.