media-db: Port to GFile and add support for URIs in the database

Add support for URIs in the database, in a backwards-compatible manner. URIs
are stored in the files table with base_path_id set to 0, and no corresponding
entry in the paths table. Normal paths or local URIs are stored split between
the files and paths tables as before.

This adds a dependency on GIO, which was already implicit due to depending on
GLib 2.30.
snra-manager: Implement serving of non-local files using HTTP redirects

Instead of proxying non-local files through the daemon (which would be the
correct thing to do, but very complex), serve them as a HTTP redirect to the
client, which can then stream them itself.

This also works for custom files requested from the server.
snra-manager: Allow non-local URIs for enqueuing custom files

Instead of just allowing absolute paths on the server to be passed as custom
files to the /control/next interface, allow full URIs. This allows clients to
request and play DLNA resources without requiring them to be in the media DB.
docs: Add some important warnings about the security of the server

i.e. The server has more security holes than a colander which has just been
put through a firing squad.
Add the ability for clients to send status messages

Display arbitrary status messages from clients in the web UI.
Merge branch 'dlna-playback' of git://github.com/pwithnall/aurena into pwithnall-dlna-playback

Merged with modification: Only allow playing http URIs, not any arbitrary
URI

Conflicts:
	configure.ac
	src/server/snra-media-db.h