Fixed Unicode surrogate pairs parsing issue

The biggest change in this version is that we have fixed a problem with parsing Unicode surrogate pairs in JSON strings, e.g., "I \ud83d\udc95 Unicode": I 💕 Unicode. There are no API level changes in this release.

First public version

This is the first public version of the spotify-json library. We intend to keep the API stable in accordance with semantic versioning. The codecs are considered part of the public API, but the encoding_context and decoding_contexts might change in the future, and is thus not part of the API. This also means that the codec interface is an implementation detail at this point.