added memcache injection plugin

This is the first version, only batch injections supported by now.
Tested on pylibmc==1.3.0.

Since it has to have send_mutant method, I believe it should be:
class RequestSendMixin(ExtendedUrllib):
    def __init__(self):
        ExtendedUrllib.__init__(self)

    def send_clean(self, mutant):
        """
        Sends a mutant to the network (without using the cache) and then returns
        the HTTP response object and a sanitized response body (which doesn't
        contain any traces of the injected payload).

        The sanitized version is useful for having clean comparisons between two
        responses that were generated with different mutants.

        :param mutant: The mutant to send to the network.
        :return: (
                    HTTP response,
                    Sanitized HTTP response body,
                 )
        """
        http_response = self.send_mutant(mutant, cache=False)
        clean_body = get_clean_body(mutant, http_response)

        return http_response, clean_body