Expose ping method on WebSocket driver.

[digitalextremist]

As requested in kenichi/angelo#52: add a forwardable delegator for ping to @driver.

[kenichi]

or maybe just add :driver to the attr_accessor list?

[digitalextremist]

@kenichi that's a possibility, I'd need to think about that. I'm leaning toward this being more of a duck type and abstraction layer, rather than encouraging people to directly access the driver and become dependent on driver-specific code some day.

Will ponder while also resolving #186, but currently over in Celluloid itself for a bit.