create metrics.expose to expose metrics to the running application, in b...

...othclojure and json formats.

Hi Steve,

I wanted to have access to the rendered clojure maps of the metrics separate from a piece of Ring middleware as was in the expose.clj file in the ring project.  

This is not quite finished, in that the tests fail because I need a way to reset the global metrics.  I wanted to ask you about that.  One look at the Java docs and I only see removing by metric name.  To be able to remove them all I'd need to keep track of them as we create metrics... but that seems heavy handed.  I wanted to be able to test the new functions I added, though, so what do you think?

Also, I am considering removing the `expose/json-metrics` so that the main metrics project won't need to depend on Cheshire.

What do you think?