Support debian unames

Without this patch, running on my debian sid system only produces

    $ ./mbox sh
    ./mbox: Bad OS release string: '3.12-1-amd64'
Ignore .o files

After typing make in src/, I get a lot of .o files lying around and showing up in git status.
This commit marks these files ignored.
Fix line wrap and link in README, and make it render as Markdown.
Merge branch 'ignore-o-files' of https://github.com/phihag/mbox
Merge branch 'fix-readme' of https://github.com/phihag/mbox