DJSON-17: Throw on infinite or NaN floating-point values

IETF RFC 7159 (JSON) does not permit Infinity and NaN as numeric
values.  http://tools.ietf.org/html/rfc7159#section-6

To render Infinity or NaN as strings, use a custom value-fn.

http://dev.clojure.org/jira/browse/DJSON-17
README: add DJSON-17 to change log