[RFC] Started initial work on sendMulti

This is just my idea of how the method should look. It should work serially for all adapters and the event system should still be called. Now the trick is to override this method in adapters that support parallel requests... Possible solution for #31 

Feel free to comment or criticize. I am not hung up on this being the way to do it.
See my "TODO" comments in the `GuzzleHttpAdapter` - I don't have access to the original request... not sure what we should do.