Release Name: 2.1.0

Notes:
- New Mixer::Music module for playing music files.
- New Sprite features: depth sorting and built-in #undraw method.
- Many other improvements and bug fixes.


Changes:
=== Features

- Add: Mixer::Music module.
- Add: Mixer.driver_name method.
- Add: Depth sorting for sprites.
- Add: Sprite#undraw.
- Add: TTF#size_utf8 and TTF#size_unicode.
- Add: Rubygame.key_name.

=== Bug Fixes

- Fix: Bug with dirty rectangles in UpdateGroup.
- Fix: Incorrect docs for joy-hat direction value.
- Fix: Segfaults when Joysticks are garbage-collected after Rubygame has quit.
- Fix: Segfaults when using Surface#convert, #to_display, or #to_display_alpha
  when no video mode has been set.
- Fix: Surface#get_at returning [0,0,0,0] when it shouldn't.
- Fix: Sprites::Group#collide_group behaving incorrectly (again).
- Fix: Build system doesn't correctly handle quoted spaces in --cflags/--lflags.
  (NOTE: Use CFLAGS/LDFLAGS environment variables now, instead.)

=== Known Issues

- Build system won't detect changed source files and recompile as necessary;
  you must "rake clobber build" after a change.

=== Other Stuff

- Added argument checks to several Rect methods.
- Surfaces can be created with no Screen mode set.
- Mixer.open_audio has default values.
- TTF style getting/setting refactored under the hood.
- Added demo_music.rb and a sample song.ogg to try out music.

=== Acknowledgements

- Johan Berntsson for adding Mixer::Music.
- Daniel Neis Araujo for adding Sprite#undraw.
- Roger Ostrander (denor) for and bug fixes and adding Rubygame.key_name.
- Bjorn De Meyer (beoran) for numerous patches and improvements.