Added simple file persisting mechanism.

[drapostolos]

Added a default persisting mechanism, that can persist the state of each PolledDirectory to file, which can later be used by another instance of the DirectoryPoller.

Usage Example:

Path storage = Paths.get("folder", "file.dat");
DirectoryPoller.newBuilder()
.addPolledDirectory(new JavaIoFileAdapter(new File("C:\\temp\\polled1")))
.enableDefaultStatePersisting(storage, 
    dir -> ((JavaIoFileAdapter)dir).getFile().toString(), 
    str -> new JavaIoFileAdapter(new File(str)))
.addListener(new AbstractRdp4jListener() {
    @Override
    public void fileAdded(FileAddedEvent event) throws InterruptedException {
        System.out.println("+" + event.getFileElement());
    }
    @Override
    public void fileRemoved(FileRemovedEvent event) throws InterruptedException {
        System.out.println("-" + event.getFileElement());
    }
    @Override
    public void fileModified(FileModifiedEvent event) throws InterruptedException {
        System.out.println("@" + event.getFileElement());
    }
})
.start()
.awaitTermination();

[drapostolos]

Extends the change in PR #26

[drapostolos]

This has been released in version 0.3.0