contents: Convert StaticContent to Static, a Page subclass

Static needs a lot of the same handling as other pages, so make it a
subclass of Page.  The rename from StaticContent to Static makes for
cleaner configuration settings (STATIC_URL instead of
STATICCONTENT_URL).

All currently generated Static instances override the save_as
attribute explicitly on initialization, but it isn't hard to imagine
wanting to adjust STATIC file output based on metadata (e.g. extracted
from their source filename).  With this union, the framework for
manipulating URLs and filenames is shared between all source file
types.
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.