Modify SerialPort to implement java.io.Closeable

Per #48 (and its dependency on #47), this patch causes `SerialPort` to implement `java.io.Closeable`. This permits the operation of general I/O-related components upon jSSC-provided serial ports. Importantly, this enables the use of Java 7 [try-with-resources](http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html) statements with serial ports.