Added diff method

[omidfarahani]

• Added diff method which will return difference between any given dates in an array containing days, months and days.
  • It should be noted that Carbon diff functions calculate date differences based on Gregorian calendar months and are not suitable for Jalali dates.
• Added getLastDayOfMonth method
• Added getLastDayOfYear method
• new methods are accompanied by corresponding test methods

[morilog]

These methods will be added in the new major version, soon.

[morilog]

Please fix your styles and use camelCase variable names

[omidfarahani]

As two new methods (getEndDayofMonth and getEndDayOfYear) did the same job which was expected of getLastDayOfMonth and getLastDayofYear, there was no need for my methods. So I closed this request and created a new one including just the diff method.