Add and_then combinator

[Marwes]

Intended to solve #22. I think the addition of the Other variant is an useful regardless but I am a bit vary of the and_then function since there is some overlap with then then function and I could also think of a third function which would have a signature like:

fn and_then<F, O>(self, f: F) -> AndThen<Self, F>
         where F: FnMut(Self::Input, Self::Output) -> ParseResult<O, Self::Input>;

[hawkw]

Nifty, thanks!