Clean up--delete and move some code, no change in functionality

Deleted some dead code, removed redundant imports, and moved a
couple of top-level functions into a where-clause.
Create Gramophone.MediaController.MediaController type

Currently the mediaController type is just used as an unforgeable
token to ensure the GStreamer library has been initialized before
and functions that need it are called.

Eventually, this type may be used for other purposes, such as
returning playback status.
Change module organization

Renames:
  Gramophone.Gramophone      -> Gramophone.Core
  Gramophone.Database        -> Gramophone.Core.Database
  Gramophone.MediaController -> Gramophone.Core.MediaController

Gramophone now has two submodules, Core and GUI, with other
modules inside core. The Core module is self-contained, exposing
all gramophone functionality as a library. Main uses GUI to
provide this functionality as an application.
Add README file.

Technically, README.md.