Automatically send heartbeat requests (ping) unless given "?ping=0"

[clue]

The automatic "pong" mechanism allows the Quassel core to detect the connection
to the client is still active. However, it does not allow the client to detect
if the connection to the Quassel core is still active. Because of this, this
project will automatically send a "ping" (heartbeat request) message to the
Quassel core if it did not receive any messages for 60s by default. If no
message has been received after waiting for another period, the connection is
assumed to be dead and will be closed. You can pass the ?ping=120.0 parameter
to change this default interval. The Quassel core uses a configurable ping
interval of 30s by default and also sends all IRC network state changes to the
client, so this mechanism should only really kick in if the connection looks
dead. If you do not want this and want to handle outgoing heartbeat request
messages yourself, you may pass the optional ?ping=0 parameter like this:

$factory->createClient('quassel://localhost?ping=0');

This is technically a BC break because consumers of this package would need to handle this manually previously and switching to this new behavior means that this consumer code may now send duplicate heartbeat requests. In practical terms, adjusting this is rather straight-forward and this makes consuming this library easier for most common use cases.

Builds on top of #38