release v0.2.0

release v0.2.0

* Improved documentation.
* Upgrade bundled ion-c library.
* Fix gh-issue #5: ion\Timestamp crashes with string timezones.
* Add ion\Exception.
* Refactor (un)serialization facilities:
  * Removed ion\Reader\Options class and moved the options to the ion\Reader constructor.
  * Removed ion\Writer\Options class and moved the options to the ion\Writer constructor.
  * Renamed ion\Serializer\PHP to ion\Serializer\Serializer.
  * Renamed ion\Unserializer\PHP to ion\Unserializer\Unserializer.
  * Changed ion\Serializer::serialize(mixed $data) to
    ion\Serializer::serialize(mixed $data, ion\Writer|array|null $writer = null).
  * Changed ion\Unserializer::unserialize(string|resource $data) to
    ion\Unserializer::unserialize(ion\Reader|string|resource $data)
* Add support for custom (un)serializers.