Can now export AIS to JSON

To make it easier to interoperate with other languages, there is now the ais2json commmand-line tool and support in the Sentence object for converting sentences to JSON. Field order is preserved to make the JSON human-readable. So that no information is accidentally lost, the original encoded AIS text is attached.

The JSON format is much more verbose, but that can be compensated for via compression. A quick test on a 2 GB raw AIS file shows that raw AIS is about 79 bytes per sentence, while a gzipped JSON version is about 64 bytes per sentence.