Sort the files gathered from `ls /etc/*[_-]release /etc/*[_-]version`. Fixes #55. Refs #54 and #56.

Since we need a custom sorting order, there's a new function to do that job, `__sort_release_files`. Sorting is first done by knowledge. If we know a release file, it goes to the primary list. If it's unknown it goes to the secondary list.
Afterwards the primary list gets sorted by maximum importance, ie, the CentOS files needs to come before the RedHat files.
Afterwards, the primary list gets sorted again, but this time, by the lowest importance, ie, the lsb-release file should come last(it's handled separately).
Finally, the function returns the primary list following the secondary list, and that will be the order for which we try to detect the distributions.