Support for $fallback with falsy values.

$value = Setting::get('setting', false);

When no setting found, $value would be `null` instead of the given fallback when the v$fallback value is falsy ('', 0, false).
Added support for default values in the FallbackInterface
Merge branch 'master' of git://github.com/bfanger/Setting into bfanger-master
Merge branch 'bfanger-master'