dwarfs-0.7.3

This is a small incremental update over the 0.7.2 release adding a single new feature: forward compatibility. This means that the 0.7.3 release will be able to handle DwarFS file system images created with newer releases as long as these images don't use features that are not understood by the older binaries. Up until now, support for new features often triggered a file system version increment, rendering the images unusable with older binaries even if the features weren't actually used in the image. This fixes #158.