Backwards-compatibility for clojure.contrib.test-is

The clojure.contrib.test-is library moved from Contrib into the
Clojure distribution as clojure.test.

This happened on or around clojure-contrib Git commit
82cf0409d0fcb71be477ebfc4da18ee2128a2ad1 on June 25, 2009.

This file makes the clojure.test interface available under the old
namespace clojure.contrib.test-is.

This includes support for the old syntax of the 'are' macro.

This was suggested by Howard Lewis Ship in ticket #26,

Vars in clojure.contrib.test-is