JsonGenerator and JsonParser should implement Closeable (and Flushable)

Implementing Closeable allows one to use try-with-resources which would be nice.

Flushable on [JsonGenerator](https://github.com/google/google-http-java-client/blob/master/google-http-client/src/main/java/com/google/api/client/json/JsonGenerator.java) isn't as needed, but it probably doesn't hurt.