Implement otherwise/0

This is the same as otherwise/0 as defined in library(quintus).
Polish the README a little
Add if/2 and if/3
Clarify why otherwise/0 is included

Most people who want otherwise just allow SWI to autoload the definition
from library(quintus).  For reasons of comfort that's not always
preferable.
Add todo/{1,2,3}
Add in/2

A generic, extensible membership relation.