Set Plus() => 0 and Times() => 1

[Axel Kramer]

If Plus or Times have no argument, they return unevaluated.

Plus() should be evaluated to 0 and Times() should be evaluated to 1.

If implemented this implies also changes to other built-in functions like for example Together()

[Axel Kramer]

Fix issue65: Set Plus() => 0 and Times() => 1

→ <<cset 41cf4b965a04>>