yesterday isn't parsed correctly

Hi,

Using ```PretyTimeParser.parse("yesterday")``` gives you back either today or tomorrow. However if you use ```PrettyTimeParser.parseSyntax("yesterday")``` you get back the date you would expect.

The issue seems to be with the ```relativize()``` logic in ```parse()``` method. As I don't know the full picture behind the relativize logic, I didn't really want to change that method, so I'm at least supplying a failing test.

If you can have a look at it, that would be great as I really like using this project!

Cheers,
Michal