Fix relative path access for rarfile

This fixes a bug so pyarrfs now runs on Debian systems.

It's really weird that it worked before on my Ubuntu system as well as
on a Debian 6.0 while it broke down on Debian 7.0.

Anyway, this was a pure and obvious bug so fixed!
Fix statfs + some more

Fix the statfs call, which is used by 'df' and friends. Since pyarrfs
doesn't really affect things like that, we merely pass it through to the
underlying filesystem.
Merge branch 'fix-relative'
Bump version to 0.8.1