usernado.helpers.api_route should be singleton

``` python
# In foo.py

from usernado.helpers import api_route
.
.
.
```

``` python
# In bar.py

from usernado.helpers import api_route
.
.
.
```

``` python
# In server.py

from usernado.helpers import api_route
.
.
.
web.Application(api_route, **settings)
```