Added some handy methods to NSDate category

I've been using these methods a lot in an app that handles events, dates, etc. Since I started using SSToolkit recently I think it would be nice to have it all in one place. I've added these methods:

    + (NSTimeInterval)unix;
    + (NSTimeInterval)dayInSeconds;
    + (NSTimeInterval)weekInSeconds;
    + (NSTimeInterval)monthInSeconds;
    + (NSTimeInterval)yearInSeconds;
    
    - (BOOL)occursToday;
    - (BOOL)occursTomorrow;

Only the last two methods have tests included.

Happy coding!