feat(unsubscribe): Add unsubscribe function

[declanelcocks]

I'm using your library for some simple real time notifications and noticed that on logout the connection was not being destroyed. As well as that, I needed to subscribe and unsubscribe from some channels based on specific variables. This function I've added works for me, but it's as basic as it can get, so feel free to add any data or callbacks to it.

I realise this repo is quite inactive, but thanks for the great angular wrapper :+1: