Set custom response class in test environment

Custom response class has `json` method to help test JSON responses:

    def test_json(client):
        res = client.get(url_for('ping'))
        assert res.json == {'ping': 'pong'}
Update version number
Add note about testing JSON responses to README
Merge pull request #3 from vitalk/custom-response-class

Extends default `response_class` to test JSON responses