Basic support for interceptors = code coverage?

This allows to define a custom `Testem` object that overrides the default one.
By doing so it's possible to define additional methods or modify testem behavior.

The extend allows to create a `Testem` object before `testem.js` is loaded.

I kept this change as light and generic as possible, but the idea behind is to support code coverage (or at least go in that direction).

I'm the author of `node-coverage` and my tool (as well as istanbul) creates a JSON object that should be sent to a server to be stored on disk after the tests are done and before the browser is closed.

With this small change on testem I can include the following script in my test page

```js
Testem.evtHandlers = {
	"all-test-results": [function () {
		sendCoverage()
	}]
}
```

This works both for `node-coverage` and `istanbul`.

The `extend` is needed in my setup because I don't have much control on the order of loading scripts, so I can't guarantee that the extra script gets loaded after `testem.js` and thus call `Testem.on()`.


The changes on `api.js` allow to do the following

```js
testem.on("before_start", function () {
   testem.app.server.on("server-start", function () {
      testem.app.server.express.put('/save-report', function (req, res, next) {
          storeReportSomehow(req.body);
          res.end("OK");
      });
   })
});
```

that basically means adding a middleware onto express server.

I thought about other ways to add middlewares, like defining them in the config or providing a hook, but it's more specific and requires few changes more.

What do you think?