Fix for handling shortnames in unlinkEntry and linkEntry

First of all I like to say that I am using fat32-lib with much pleasure. During some development I also found the bug described in issue 14 and saw that there already was a fix for it.
I looked at the fix and noticed that makeShortname also uses usedNames and that after calling unlinkEntry these shortnames are still in the usedNames set and that after linkEntry is called the usedNames is missing the shortname for the new entry.
This I fixed and also added a test for it.