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.