Create a persistent connection that can be used to send multiple push notifications.

This cleans up the implementation in client.go and supersedes it. However, for backward-compatibility, client.go will remain for a while. Apple treats a lot of connects/disconnects as a potential DoS attack; since nobody knows the threshold, you should use one connection to send multiple notifications.

The connection also allows the timeout to be configured by the user; it defaults to the TIMEOUT_SECONDS constant but provides a SetTimeoutSeconds method.
Make the connection timeout uint8 instead of int8.

Your negative timeouts are not welcome here.
Update the README with instructions on the new connection behaviors and options.
Fix a typo in a comment.
Merge pull request #1 from btmerr/persistent-connection

Persistent connection