Fix Bidi.

That seems to fix #42. The issue was that the `bidi_classes` array was initialized in the constructor (instance method), but was used in `bidi_class_for` (class method).

The lesson is that anything that is not tested is either already broken or will eventually break :( I added a TODO and will create an issue for adding more tests for `Bidi`. I'd prefer to add them right away, but I'm not really familiar with the API and the implementation, so it might take me some time to add proper tests.

@israelidanny, it'd be great if you could check out this branch and confirm that it's working for you. As I said, I'm not exactly familiar with this code, so I just made sure that the example from README works. If that was the only bug in there, Bidi should be fixed. Otherwise I'll need to take another look at it.