adding json-spec reporter

The intended use of this reporter is to make it possible for a hybrid of manual/automated tests to be executed.  In some cases, we want to be able to run tests and observe the results in real time via the `spec` reporter, but we also want to be able to capture the results in `json` so that they can be archived and reported on in our CI system.

The `json-spec` reporter allows for this by having the `spec` output sent to stderr and the `json` output sent to stdout.  Exit codes are left unaffected.  This makes it possible to run tests like this:

```
$ mocha -R json-spec >> ~/testresults/mytest.json
/* stderr =>
 * {{ spec reporter output here }}
 */
$ cat ~/testresults/mytest.json
/* stdout =>
 * {{ json output here }}
 */
```