Use custom logger if configured, fallback to Rails.logger/STDOUT only if not

According to docs, you can set custom logger for APNs
```
APN.logger = Logger.new(File.join(Rails.root, 'log', 'apn_sender.log'))
```
But it's overwritten by Rails.logger (and it's cool beacause heroku deploys won't work otherwise, as discussed here https://github.com/arthurnn/apn_sender/issues/56). The drawback is that you cannot specify custom logger any more. So this patch should add usage of defined one, and fallback to Rails.logger/STDOUT if not.
What do you think?