Detection and reporting of unrar binary version, and rar version of rar files

Detection and reporting of unrar binary version, and rar version of rar files, as discussed on https://forums.sabnzbd.org/viewtopic.php?f=4&t=17490

Example output in sabnzbd.log:

2014-05-07 21:40:31,845::INFO::[SABnzbd:484] unrar binary version is: UNRAR 5.00 beta 8 freeware
...
2014-05-07 22:06:32,256::DEBUG::[newsunpack:599] Type of rar file is Rar4 or lower

To avoid code duplication, it also possible to tweak sabnzbd.utils.rarfile.is_rarfile() so that it returns None (for not a RAR) or the version number.