support `files` with IPython 1.0

This simply disables the `strip_files_prefix` filter,
since we are detecting it and using it for redirects.

We will need to think more carefully when the `files/` prefix
is no longer in files to begin with.
Merge pull request #106 from minrk/files-1.0

support `files` in 1.0

skips the strip_files_prefix filter, to allow the redirect to continue to work