Correct usage of NSDateComponents

Hi Erica,

I noticed some smalls bugs in this extension.

* isSameMonthAsDate only asked for the NSYearCalendarUnit but uses the month property when comparing too.
* isTypicallyWeekend extracts the year property but needs the weekday property when comparing.

I also simplified (I think this is faster as well) the different decomposing methods, by only using the components required in the method call.