digest command

This implement the `digest` CLI command using the DigestsFile and
AssetFile handler classes.  You can pass file path(s) to digest in
as arguments or pass nothing and digest ever file  in the config's
`files_path` tree.

The digest cmd also manages the paths used in the digests file.  Each
path is relative to the `config.files_path`.

@jcredding ready for review.  you have any suggestions on better handling for the relative path junk.  the logic is spread across both the AssetFile and the DigestCommand classes - blegh.