Adding some extra functions

Hello!

I got some ideas for some functions. I have programmed and tested 17 new function:
- replaceItems
- properties
- numberOf
- duplicates
- remove
- clear
- move
- keep
- sum
- mean
- delta
- removeItems
- unify
- in
- AND
- OR
- propertyContains

(All the functions is described as comments in the source)

Maybe some of them are notable?