added memcache injection plugin

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

This function is also defined in `blind_sqli_response_diff.py`. I don't like code duplication, we need to solve this. A potential solution would be to define a class called `RequestSendMixin` and have `blind_sqli_response_diff` and `AuditPlugin` inherit from it.