Implemented encoding for strings

* Packer by default uses `utf-8` encoding by default
* Unpacker uses `None` by default, so no decoding is done
* Both pack and unpack has `encoding` and `unicode_errors` arguments,
  if `encoding` is `None` no encoding/decoding is done, otherwise
  it is python codec. `unicode_errors` is supplied as `errors`
  parameter to codec

python/test3/test_obj.py

@@ -26,7 +26,7 @@ def test_decode_hook():
     unpacked = unpacks(packed, object_hook=_decode_complex)
     eq_(unpacked[1], 1+2j)
 
-@raises(TypeError)
+@raises(ValueError)
 def test_bad_hook():
     packed = packs([3, 1+2j], default=lambda o: o)
     unpacked = unpacks(packed)