[compat] Add proxy property to pydle.Client.PING_TIMEOUT for reverse compatability

[Joshua Salzedo]

#124 renamed pydle.Client.PING_TIMEOUT to pydle.Client.READ_TIMEOUT, which may effect downstream users as Pydle's public interface changed.
I suspect this change was named to keep the variable appropriately named, however for the sake of compatibility with existing code we should preserve the old name too, for the time being.

What i propose is a proxy property using the old name, which will log a deprecation warning in Pydle 0.9.3 onwards. At some point down the line, possibly 0.10.0 or later, this proxy can be removed.

This will allow old code to continue to work, while giving developers a warning so they can migrate existing code to the new variable