refactors GenerateOggSerial into a generic GenerateSerial
utility, under the util lib.

libFLAC may be encoded without Ogg support. If Ogg support is disabled,
libFLAC will still export Ogg-related methods (like setting a serial
number), and throw a runtime error when initializing an Ogg stream.

GenerateOggSerial does not depend on libogg. Refactoring it into
a generic GenerateSerial prevents having to add build-time checks
for libogg within the FLAC encoder plugin.

This is semi-standard and doesn't require the util/Compiler.h header.

This allows using it in constant expressions.

This makes it compatible with containers storing std::unique_ptr.

May allow the compiler to optimize calls away.

Add support for the following tags when using the proxy database plugin:
WORK
CONDUCTOR
LABEL
GROUPING
MUSICBRAINZ_WORKID
COMPOSERSORT
ENSEMBLE
MOVEMENT
MOVEMENTNUMBER
LOCATION

Upcoming release of game-music-emu will support it, details here: https://bitbucket.org/mpyne/game-music-emu/pull-requests/23/rsn-support

The current http output doesn't provide a header for cross-origin support. This prevents to use the mpd http stream directly from an other webapplication due the origin from the webpage differs from then the audio stream.

The fix is to add the following header to the http response:
Access-Control-Allow-Origin: *

This commit adds some tags that are (mostly) interesting for listeners
of classical music.

Ensemble
--------
This is an ensemble that is playing the music, such as Wiener
Philharmoniker. The tag can be used to distinguish the ensemble from the
conductor, composer, soloist, and ensemble, that are generally all in
the "ARTIST" tag.

Movement
-------
The movement number and movement (name) of this track, i.e.  "II" and
"Allegro".

ComposerSort
------------
Allows us to look for Beethoven's 9th under B, for Beethoven, not L for
Ludwig.

Location
--------
This is the location of the recording, e.g. "Wiener Musikverein".