provides implementations of ex-info and ex-data for slingshot to
use with clojure versions earlier than 1.4.0

ex-info and ex-data are currently scheduled to be available in
clojure.core starting with release 1.4.0. These implementations are
based on that code (issue CLJ-733), using slingshot.ExceptionInfo in
place of clojure.lang.ExceptionInfo.

This allows slingshot to be compatible with previous versions of
clojure while also fully supporting clojure.1.4.0 and whatever tools
may be created based on clojure.lang.ExceptionInfo.