Add querystring option to skip checks

You can skip any number of checks, by passing ``?skip=module.path.to.callable&skip=...``
in the request URL. Only the checks in ``WATCHMAN_CHECKS`` which are not in the
querystring should be run, eg::

##### Request
    curl -XGET http://127.0.0.1:8080/watchman/?skip=watchman.checks.email_status

##### Response
    {
        "databases": [
            {
                "default": {
                    "ok": true
                }
            }
        ],
        "caches": [
            {
                "default": {
                    "ok": true
                }
            }
        ]
    }