Initial pure JDK implementation

[DavyLandman]

On-and-off I've been working on a nice reference implementation of a file changes watcher. Both the raw api, and the tests I could come up with.

The future will bring windows & linux (and maybe osx) specific implementation that can benefit from the native kernel api.

todo:

• add license
• setup deployment to somewhere like maven.
• add code examples to the readme
• review log levels

[DavyLandman]

@sungshik thanks for the thorough review, I think I've address all comments.

[DavyLandman]

I'm still doubting the exact interface of the Watcher class though, curious on your thoughts.

[sungshik]

I think the interface of the Watcher class is much clearer now; well done! My only suggestion is to rename the enum values a bit (but this is really a matter of taste, so just see how you feel about it). Longer term (not this PR), it could be nice to consider abstracting away also the usage of java.nio.file.Path in the public methods of Watcher and use, for instance, Rascal(-style) locations (if there's a use case for this, but I imagine there could be).

I also had a look at the other changes, and it all looks fine, except I think there are still races in BundledSubscription. I tried to come up with a fix, but it's tricky to do it without bigger synchronization blocks. Here's a proposal. The correctness guarantee I can give is: I'm not convinced it's wrong 😉. Also, with more effort, it might be possible to make the synchronized blocks a bit smaller (e.g., pull active.remove and active.add outside).

[sungshik]

Had a look at the tests. Just a few small things 👍

[sungshik]

There's also a method deleteAllFiles in class TestDirectory

[sungshik]

And a review of the non-test code since last review. Looks good!