use DateDelta type to represent deltas between dates

* change Days to contain a u32
* addition of DaysDelta to NaiveDate
* changes to Iterator Implementations
* minor docs fixes

Author: esheppa

src/lib.rs

@@ -472,7 +472,7 @@ pub use format::{ParseError, ParseResult};
 
 pub mod naive;
 #[doc(no_inline)]
-pub use naive::{Days, IsoWeek, NaiveDate, NaiveDateTime, NaiveTime, NaiveWeek};
+pub use naive::{Days, DaysDelta, IsoWeek, NaiveDate, NaiveDateTime, NaiveTime, NaiveWeek};
 
 pub mod offset;
 #[cfg(feature = "clock")]