Gutter theme overhaul...

- Gutter themes can now be located anywhere. They are located by looking for <name>.gutter-theme, which can contain JSON meta-information, such as "colorize" for themes that need to be colorized. This allows gutter themes (and gutter theme collections) to be installed as packages.
- Added "gutter_theme_excludes" to settings, allows names to be filtered.
- Reworked existing gutter themes to use new scheme.
- Culled some gutter themes.
- Removed resource fork from some images.