added automatic url_rule generation in route()-decorator

I'm not sure if this is something you/we really want so I only updated the code. If you like it, I will add tests and update the docs.

With this change the following is possible:
```python
@plugin.route(is_root=True)
def root():
    items = [
        {'label': 'Latest',
         'path': plugin.url_for('show_movies')},
    ]
    return plugin.finish(items)

@plugin.route()
def show_movies():
    items = [{
        'label': movie['title'],
        'is_playable': True,
        'path': plugin.url_for(
            endpoint='play_movie',
            movie_id=movie['id']
        )
    } for movie in get_movies()]
    return plugin.finish(items)

@plugin.route()
def func_with_much_params(foo, bar, lorem, ipsum):
    pass

@plugin.route()
def play_movie(movie_id):
    url = get_url(movie_id)
    return plugin.set_resolved_url(url)
```

The URLs from the above example will be:
```
'/'
'/show_movies'
'/func_with_much_params/foo/<foo>/bar/<bar>/lorem/<lorem>/ipsum/<ipsum>'
'/play/movie_id/<movie_id>'
```

I could also be changed to _not_ add the param name, example:
```
'/'
'/show_movies'
'/func_with_much_params/<foo>/<bar>/<lorem>/<ipsum>'
'/play/<movie_id>'
```

This change should be fully backward compatible.
Personally I prefer defining the routes myself but while in development it saves some time. ;-)