Avoid throwing a KeyError when scaling down to 0 dynos

Hello!

I believe I have a patch for issue #10 - throwing a KeyError when scaling down to 0 workers. Interested?

--

test case:

1. scale down to 0 dynos if >= 1 currently exist

	HEAD branch behaviour:

		- Process is scaled down to zero
		- Trying to return 'self.app.processes[self.type]' throws a KeyError:

			  File "/app/.heroku/venv/lib/python2.7/site-packages/heroku/structures.py", line 46, in add
			    return self[0].new(*args, **kwargs)
			  File "/app/.heroku/venv/lib/python2.7/site-packages/heroku/models.py", line 510, in new
			    return self.app.processes[type]
			  File "/app/.heroku/venv/lib/python2.7/site-packages/heroku/structures.py", line 86, in __getitem__
			    raise why
			KeyError: 'web'

	After patch:
		- Process is scaled down to zero
		- An empty ProcessListResource is returned (is that acceptable?)