add PingCheck

[david-d-h]

This PR adds the PingCheck, it can be used to check if your application can reach other hosts. It can be configured like this

OK::checks([
    PingCheck::config()
        ->address('www.google.com')
        ->maxTimeout(100), // milliseconds
]);

If they are not set, address and the max timeout will be set to www.google.com and 100 by default respectively.

[markvaneijk]

Lekker bezig @dulkoss!