default.nix: latest GHC is 7.8.3
cabal: sort and liberalise dependencies

Organise dependencies into platform and additional
dependencies.
Also set upper bounds to major versions only.
Handle cases where the output has no mixer

stVolume is now a Maybe, where Nothing indicates that
the output has no mixer support.
Wrap stBitrate and stTime in Maybe

Providing a value for either when no song is
playing makes no sense.
Add Unparse instance for Maybe