Add support for ratio

[r0man]

Hi @plexus,

This PR adds support for clojure.lang.Ratio fixing issue #24.

Since Elisp does not have a ratio type, a Clojure ratio is converted
to Elisp using this expression:

(/ numerator (float denominator)).

We are calling float on the denominator to perform floating point
arithmetic, instead of the default integer arithmetic.

I was thinking about introducing a new tagged type called :ratio, but
since clojure.lang.Ratio is a sub class of Number in Java as well, I
extended the lexer and parser that handles numbers.

public class Ratio extends Number {}

WDYT?

[plexus]

Thanks a lot for contributing this! The approach seems reasonable. It's unfortunate that it's lossy, but in most situation getting an actual number back will be what people expect. I think preserving it as a tagged type can be a future enhancement, as an option.

[r0man]

Perfect, thank you!