@smoketest decorator identifies smoke tests

This story is a first (and easy) step to #23. It may be enough, so let's try it.

The `@hospital.smoketest` decorator marks objects as `is_healthcheck` and `is_smoketest`. So it will be possible to collect and run only smoketests.

``` python
import hospital

@hospital.smoketest
def test_http_200():
    hospital.assert_http_response('http://example.com', status_code=200, timeout=1)
```
