Add getter and comparison to NSDate

[PGLongo]

Hi,

I have added getters to NSDate. Now you can do:

let date = NSDate()
let year = date.year

instead of

let date = NSDate()
let calendar = NSCalendar.currentCalendar()
let components = calendar.components(.CalendarUnitYear, fromDate: date)
let year = components.year

I have also added Comparison operator: <, <=, >, >=, == and updated the docs and README.md

[pNre]

Hi, before merging, there's a bug in the > operator implementation you should fix.

public func >(lhs: NSDate, rhs: NSDate) -> Bool {
    return !(lhs < rhs)
}

If lhs == rhs the function returns true.

[PGLongo]

Thanks!

Fixed replacing

public func >(lhs: NSDate, rhs: NSDate) -> Bool {
    return !(lhs < rhs)
}

with

public func >(lhs: NSDate, rhs: NSDate) -> Bool {
    return !(lhs <= rhs)
}

I have also updated the test

[michaeleisel]

You should only implement == and > (or maybe <, I forget, but just one of the two). Swift infers the other variants, e.g. >=, for you.

[michaeleisel]

@PGLongo @pNre