Generate authorize_url.

        >>> GitHub(client_id='3ebf94c5776d565bcf75').authorize_url()
        'https://github.com/login/oauth/authorize?client_id=3ebf94c5776d565bcf75'

def authorize_url(self, state=None):
        '''
        Generate authorize_url.

        >>> GitHub(client_id='3ebf94c5776d565bcf75').authorize_url()
        'https://github.com/login/oauth/authorize?client_id=3ebf94c5776d565bcf75'
        '''
        if not self._client_id:
            raise ApiAuthError('No client id.')
        kw = dict(client_id=self._client_id)
        if self._redirect_uri:
            kw['redirect_uri'] = self._redirect_uri
        if self._scope:
            kw['scope'] = self._scope
        if state:
            kw['state'] = state
        return 'https://github.com/login/oauth/authorize?%s' % _encode_params(kw)