@smoketest decorator identifies smoke tests

[benoitbryon]

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.

import hospital

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

[benoitbryon]

Moved to milestone 0.5.