Update library to work with current version of Phoenix

[joelekstrom]

I'm sorry I haven't submitted this earlier, but here goes:

This updates the lib to work with the current version of Phoenix. For example, it's now called phx_join instead of join, and also all messages now need a unique number called ref.

Some crashes related to non-nilled pointers were fixed as well.

Thanks for the library!

[joshdholtz]

@accatyyc Please don't be sorry at all 😊 You got it done faster than I did 🙌 I will play with this later today! Thanks so much!

[joelekstrom]

Oh that's right - I also added a mechanism to handle message replies with a block callback.

- (void)sendEvent:(NSString*)event payload:(id)payload onReply:(HandleReplyBlock)handleReplyBlock;

This might actually break compatibility with current clients. It's easily fixed by adding back the same method without the last argument though. I can fix that if you feel that it's important!