Separates handler methods for commands and reqres

Fixes #21 

`Wreqr.Handlers` is no longer a class but instead a collection of methods. This is done so that its methods can be aliased by the commands and reqres objects.

Instead of

```js
x.commands.setHandler();
x.reqres.setHandler();
```

You can now merge the messaging systems *directly* on the object to write

```js
x.setCommand();
x.setRequest();
```

This is more in line with how Backbone attaches Backbone.Events onto objects.

**Note:** ~~There should be more tests added with an object that has all three merged together, and~~ there might be some improvements to the implementation. This is just an introduction to the idea.