README: Add additional build dependencies

Java and wget are now dependencies as well. Don't be surprised to see us
download the build system!

Signed-off-by: Bart Kuivenhoven <bemkuivenhoven@gmail.com>
vfs: add slab allocator and alter open symantics

VFS now caches and the open symantics don't require the stream to be
coupled to a URI.

Signed-off-by: Bart Kuivenhoven <bemkuivenhoven@gmail.com>