extend Closeable interface in SparkeyReader/Writer

this should allow the interfaces to be used in try-with-resources blocks in Java 1.8:

    try (SparkeyReader reader = Sparkey.open(file)) {
        ...
    }
You should also remove the close() on line 58 since it's already defined in Closeable.
We might also need to change the IOException thrown by close() to some sort of RuntimeException