Added ping interval configuration.

This adds a new config: default_ping_interval

It also adds the option to override this value in the ConnectionAdapter, allowing the ping interval to be set on a per-client basis, while keeping the same 10 second default. Ping can also be disabled altogether by defining a value < 1.

This can be useful in use-cases where you have different clients (desktops and mobiles) where battery might be an issue for some, while delay might be important to others.

Anyway, this is useful to me, but I'm not sure about others. Feel free to discard if you think it's too great a change for a small use-case. :D