Fedora support

Inspired by @korakinos Arch Linux support. I have done the necessary to get the scripts working under Fedora Linux. I have tried to make my changes fit with those he proposed for Arch Linux support to ease merging (e.g. use `gpasswd` instead of `adduser`).

I have plenty of colleagues at work using Fedora, but haven't been able to heartily recommend autosshfs due to the modifications required. No longer :-)

* I have tested this branch with Fedora 19 (beta)
* The changes should not cause a regression in the Debian/Ubuntu behaviour.