Fixes #14

It allows to call `cuetag` like so:

    cuetag Björk-Volta.cue *.ogg ARTIST ALBUM TITLE YEAR=2007 GENRE=electronic TRACKNUMBER

 which accomplishes two things:

  1. it pre-selects the fields and specifies their order;

  2. it hardcodes two fields to specific values for all tracks.

 Since the field list is hardcoded for MP3, any fields specified on
 the command line are effectively ignored. This could probably be
 implemented better, but I do not care about MP3 at all.

Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=655078
From: martin f krafft <madduck@debian.org>
Author: Dmitry Smirnov <onlyjob@member.fsf.org>

Ensures that tracknumbers are zero-padded to two digits.
That seems to be common usage.

Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=655079
From: martin f krafft <madduck@debian.org>

This avoids truncating long titles/albums/etc.

Author: Felipe Sateler <fsateler@debian.org>
Reviewed-By: Dmitry Smirnov <onlyjob@member.fsf.org>
Thanks-To: Rogério Brito.
Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=676478

 * Look for `mid3v2` first and fallback to `id3v2` according to their availability.
 * Added error message if neither of them found.