redirect 404s for "files/localfile" urls to a logical guess

This should let static-file serving work for some simple cases.
The files do not travel through nbviewer, it is a simple redirect.

if the first occurrence of `files/` is not the local-file shortcut, it will be wrong.
Merge pull request #29 from minrk/localfiles

redirect 404s for "files/localfile" urls to a logical guess

This should let static-file serving work for some simple cases. The files do not travel through nbviewer, it is a simple redirect.

if the first occurrence of files/ is not the local-file shortcut, it will be wrong - but this will only come up for urls that are already giving 404