Use Log::log or equivalent instead of Log::add

Now that `Log::add` is deprecated, use `Psr\Log\LoggerInterface`
methods instead, Log::log or the equivalent, appropriate methods

Also, when we are forcing the logs to be written, we are
testing if `Kohana::$log` is instanceof Logger. Otherwise
the `write` method might not be available in other logging
systems that are still compatible with `Psr\Log\LoggerInterface`
Relax log requirement constraint

We do not need to force `Kohana::$log` to be instanceof `Log`
anymore as the core is now Psr\Log\LoggerInterface compatible