ISO-8859-16 charset/decoder/encoder

[Charles Oliver Nutter]

Again, apologies if this is not the right list.

OpenJDK does not currently contain support for encoding/decoding
ISO-8859-16. A JRuby user recently reported that they needed this
encoding. So, I implemented it.

Here's a link to the charset/decoder/encoder in JRuby's repository:

https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/util/encoding/ISO_8859_16.java

I tried to follow the general form of the other ISO-8859 charsets, but
there's probably more reuse possible (I believe they use sun.nio.cs
classes I can't access).

Happy to contribute this. Unsure of the process.

- Charlie