Replace extra_files_path with files_path.

Bjoen: As far as I know, that is now the recommended way. And
extra_files_path seems to be broken in conjunction with the
object store.

Peter: Confirmed with @jmchilton on pull request #44 discussion
that this is now the preferred way to do this.
A few more .extra_files_path changes + README

These may not have existed when Bjoern original started work on #44.