added memcache injection plugin

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

Well, subclassing ExtendedUrllib is not a good idea since in w3af we try to have only one instance of the ExtendedUrllib object. **But** your idea actually made me think and we should add the `send_clean` method to `ExtendedUrllib`.

So, you should:
 * Add the method to ExtendedUrllib
 * Use that method from your new plugin
 * Use that method from blind sql injection detection
 * Remove the old one from blind sql injection detection