BACKENDS: Add zip filesystem node

This exposes a zip archive as a file system according to
the ScummVM common API.

The benefit of this is that generic code can use compressed
zip archives transparently without having to special case for
it.

To elaborate a bit more on why I'm doing this - it's a part of the Windows Phone 8.1 port I'm working on
and there's certain restrictions what an app is allowed to gain access to on the file system.
In particular, to be able to read files from the SD-card, the application needs to register a set of
file extensions that it is allowed to read.
With the wide plethora of different types of files that can exist in the game data (to not mention files
without extension), the most viable option I saw was to add support of containing game data in an archive.

To give an idea how the zip-fs backend is thought to be used, I pushed a proof-of-concept commit
that make the posix backend use it here:
https://github.com/skristiansson/scummvm/commit/f808db9dd3cffd4d6a4e3c3d22d5feab813e5286

I realize that pulling in code without current users might be undesirable, but this was self-contained
enough from the rest of the wp 8.1 port, so I thought I'd take a chance to get some input on this while
working on the rest.