Support for transcoding Marc8 to UTF-8

Ported from pymarc, thanks so much to the awesome pymarc developers, including asl2, edsu, gsf, and anarchivist, For for writing clean MARC8 transcoding code, with good test cases, and giving me hints in IRC. 

This does involve adding one more gem dependencies to ruby-marc -- the `unf` gem for unicode normalization. 

See the updated character encoding explanatory [docs at head of MARC::Reader](https://github.com/ruby-marc/ruby-marc/blob/marc8/lib/marc/reader.rb) for an overview. 

The work-horse class is [MARC::Marc8::ToUnicode](https://github.com/ruby-marc/ruby-marc/blob/marc8/lib/marc/marc8/to_unicode.rb) -- all actual MARC8 transcoding logic is encapsulated there, supporting re-use outside of MARC::Reader.