Add Marionette.deprecate

Resolves #1792.

We should talk about what we would like to deprecate in future. See: https://github.com/marionettejs/backbone.marionette/issues/1792#issuecomment-54090366
We might be able to use more structure here. Moment.js has a good message, I think. Maybe something like:

```js
Deprecation warning: ___ is scheduled to be removed in v3.0.0. Please use ____ instead.
```

It's a bit more informative than just yelling *'DEPRECATION!'* at the user, I think.