Module export format changed to Fetchr Class

Breaking changes:

1. Fetchr now exports the class itself, rather than the class creating factory function.

   var Fetcher = require('fetchr')(), //Fetcher class
       fetcher = new Fetcher(); //Fetcher instance

   now becomes

   var Fetcher = require('fetchr'), //Fetcher class
       fetcher = new Fetcher(); //Fetcher instance

2. addFetcher method renamed to registerFetcher. This is to keep the api somewhat consistent between fetchr and dispatchr.

3. Fetcher.middleware() should no longer be mounted directly on the app. It should always be mounted on a path.

   app.use(Fetcher.middleware());

   now becomes

   app.use('/api', Fetcher.middleware());

4. pathPrefix config option renamed to xhrPath.

   • This config option used to be passed into the class creator function, but is now passed into the class upon instantiation.
   • This option's value should point to the exact path the Fetcher.middleware() was mounted on.