Updated CSSMin, Fixed Assets::js() 1st argument, Added $prepend to Assets::add_css(), Removed $type argument from Assets::generate_file()

- Assets::js() was handling first argument by casting it to a string and
passing it to external_js() if it was not empty, even when it was an
array, without regard to the $type argument
- Added $prepend argument to Assets::add_css() to control whether CSS
assets are added to the beginning or end of the array, makes it more
consistent with add_js(), which may be useful later...
- Removed $type argument from Assets::generate_file(), which was unused
and confusing, since $file_type already handles the type
- Updated CSSMin to 3.0.1