Add Poller

This solves #52. To add to the discussion in #52, a poller is nice to have since it allows reading multiple sockets from a single task. I think this is an advantage since sockets are not thread-safe. Additionally, this makes it easier for users to replicate the examples in the guide. To the best of my knowledge this implementation is safe since the socket reference is preserved while polling, but this is far from my comfort zone so a second pair of eyes is certainly welcome.