Timestamptz lacks arithmetic (and more) operations

[KeenS]

As described in the document both timestamp and timestamp with time zone can be operated with arithmetic operations, but diesel implements Add and Sub only ofr Timestamp. Similarly, as reported in #1514 , now should be able to be treated as timestamp with time zone.

I'll implement it, if it is acceptable.

[weiznich]

Feel free to take it 👍

[KeenS]

:)

[notryanb]

Resolved in #1657