Issue #26

rewriting manifest, deletes digest-files and entries, adds non-digest entries
cherry-picked 1588a045798c7d9620ffbbb4272ecfb1a150a876

concerning issue #26
Can this line be replaced with `@filename`. The initializer for the `Manifest` class allows for passing a completely different manifest name.

https://github.com/sstephenson/sprockets/blob/master/lib/sprockets/manifest.rb#L18-L54