make default_key transliterate non-ASCII characters

Hello,

I ran into a problem with the generation of default_keys when dealing with names containing German umlauts. The following code, e. g., wouldreturn "m-a" as key instead of "muller-a" or "mueller-a".

    e = BibTeX::Entry.new
    e.author = "Christian Müller"
    e.parse_names
    e.key

My patch fixes this by changing the default_key method to transliterate the name using the iconv gem (meaning I only tested it under Ruby 2.0 where iconv has been removed from stdlib).

Im not entirely sure if this patch is useful since I do not know if bibtex-ruby should handle encodings at all or leave that task to its users. Anyway, I wanted to let you know.