add encodings support to unpack (compatible with msgpack-jruby)

[technorama]

No description provided.

[frsyuki]

Why do you need this option? msgpack spec defines String type uses UTF-8: https://github.com/msgpack/msgpack/blob/master/spec.md#types

[technorama]

1. For compatibility with msgpack-jruby which accepts an identical encoding option.
2. For sending binary data. Without this patch decoding doesn't give you the same string you encoded.
3. The spec mentions sending binary data. This should be allowed.
4. Provides a way for users to workaround bug Automatic UTF-8 string encoding in Ruby 1.9 can cause unexpected results #7
5. Provides a way for users to workaround bug Unpack strings to ASCII-8Bit, not UTF-8 #40
6. Provides a way to use this library with binary data until the spec handles binary data types/tagging in an unambiguous way.

[tagomoris]

Now we are merging msgpack-jruby into this repository. So, I think that we don't have to take care about compatibility with it.
Please reopen this again if you have any concern about it.