Allow getting mocked requests

[etandel]

Sometimes it is useful to get the requests that were made in order to check for certain arguments such as header and payload. Currently, aioresponses lets us do this by indexing into aioresponses.requests with a (method, url). However, I find this a bit clunky to use, especially when the mocked url is a regex.

This PR includes a new way of getting the requests by using the RequestMatch object that is created when defining the mock. This is done by making the mocking methods return the created RequestMatch and by adding a new .matched_requests() that takes this match objects and uses it to search for matching requests on aioresponses.requests.

Example:

        loop = asyncio.get_event_loop()
        session = aiohttp.ClientSession()

        matcher = m.post('http://example.com', status=500)
        m.post('http://example.com', status=200)

        loop.run_until_complete(
            session.post('http://example.com', json={"a": 1})
        )
        loop.run_until_complete(
            session.post('http://example.com', json={"a": 2})
        )

        requests = aioresponses.matched_requests(matcher)
        assert requests[0].kwargs["json"] == {"a": 1}
        assert requests[1].kwargs["json"] == {"a": 2}

This is my first PR to this project, please let me know if there is any way it can be improved.

[etandel]

(will fix tests)