Added initial support for loading beatmap skins.
- Added a GameImage enum for more organized loading of image resources.
- Game image loading now takes place directly before each beatmap is loaded.
- Added option 'IGNORE_BEATMAP_SKINS' to disable this feature.

Other changes:
- Slight correction in readme file: apparently the JAR will not run in the osu! program folder.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>