Version 0.4.0 release
This is minor release of API and FFI crates for libraries in alsa-gobject.
They are first version of them depending on glib crate v0.15, thus it got
tons of changes internally. For public API, some trait methods are
renamed following to fashion of the glib crate.
Documentation is added by importing from gir files of libraries in
alsa-gobject v0.3.0.
All of API crates get top-level prelude module as a way to access to all
of traits and their implementations, while while modules with the traits
and implementations becomes private.