Added a #url() method to the Resource.

This abstracts the url creation in _request and exposes it as a public API.

For saving entities to a database, templates, debugging and to avoid building URLs manually or using a different mechanism, I've found the need to expose the underlying URL slumber generates.  Using the same API just feels nicer :).

Example:
```PYTHON
>>> api = slumber.API("http://endpoint.com/api/v1/")

>>> api.entity(1).url()
   "http://endpoint.com/api/v1/entity/1/"

>>> api.entity.url()
   "http://endpoint.com/api/v1/entity/"
```

Thanks!