WFS3 client: add api() function

[justb4]

As per WFS3 spec:

in OWSLib 0.17.1 found function missing.

wfs300.py should add function like:

    def api(self):
        """
        implements Requirement 3 (/req/core/api-definition-op)

        @returns: api object
        """

        url = self._build_url('api')
        LOGGER.debug('Request: {}'.format(url))
        response = requests.get(url, headers=REQUEST_HEADERS).json()
        return response

[justb4]

PR upcoming