Monads, mo problems

I originally wrote this library while reading a book about functional programming. This book referred to things like Just and Maybe as "functors". While technically correct, the more common term for them is Monads so I've renamed the base class & updated the docs. If you use the implementation classes directly, you shouldn't have to change any code. If you inherit from Functor, you'll need to update your class to extend Monad instead.