Fix 64-bit tagged pointer issue.

[devxoul]

Idea from: sakkaras/SKSTableView#2

[nubbel]

Thank you @devxoul!
I'm afraid I cannot merge your changes, because the date components would be shared across all (SexyDates-)NSNumber instances.
Consider this case:

NSNumber *oneWeek = @(1).week;
NSNumber *fourMonths = @(4).months;    
NSDate *oneWeekAgo    = oneWeek.ago;
NSDate *fourMonthsAgo = fourMonths.ago;
NSLog(@"dates: %@", NSDictionaryOfVariableBindings(oneWeekAgo, fourMonthsAgo));

Output:

dates: {
    fourMonthsAgo = "2014-01-08 12:51:07 +0000";
    oneWeekAgo = "2014-01-08 12:51:07 +0000";
}

Note that oneWeekAgo and fourMonthsAgo result in the exact same date which is one week and four months ago. This is kind of unexpected behavior, isn't it?

[devxoul]

I didn't test enough. Sorry. I just re-sent pull request.