Add get and set bucket CORS methods

[osier]

The method to delete cors is missed, others looks good to me.

[osier]

Please add the method to delete cors. Thanks.

[markduan]

I just add the method to delete CORS, please accept this pull request.

[pengsrc]

Tests for those changes are missing, I will check that.

[pengsrc]

@markduan, for your reference

def test_bucket_get_cors(self):
    body = {
        "cors_rules": [
            {
                "allowed_origin": "http://*.qingcloud.com",
                "allowed_methods": [
                    "PUT",
                    "GET",
                    "DELETE",
                    "POST"
                ],
                "allowed_headers": [
                    "X-QS-Date",
                    "Content-Type",
                    "Content-MD5",
                    "Authorization"
                ],
                "max_age_seconds": 200,
                "expose_headers": [
                    "X-QS-Date"
                ]
            }
        ]
    }
    self.mock_http_response(status_code=200, body=json.dumps(body))
    acls = self.bucket.get_cors()
    self.assertDictEqual(acls, body)

def test_bucket_set_cors(self):
    self.mock_http_response(status_code=200)
    ret = self.bucket.set_cors({
        "cors_rules": [
            {
                "allowed_origin": "http://*.example.com",
                "allowed_methods": [
                    "PUT",
                    "GET",
                    "DELETE",
                    "POST"
                ],
                "allowed_headers": [
                    "*"
                ],
                "max_age_seconds": 400
            }
        ]
    })
    self.assertTrue(ret)

def test_bucket_delete_cors(self):
    self.mock_http_response(status_code=204)
    ret = self.bucket.delete_cors()
    self.assertTrue(ret)

[markduan]

@prettyxw check it again, please.