Enable log level switching through ssh commands

[nllptr]

The gerrit command gerrit logging set-level should be able to change the logging level of ReviewAssistant.

According to the SLF4J user manual, using LoggerFactory.getLogger(ReviewAssistant.class), creates a logger named "ReviewAssistant". The gerrit command has a parameter for setting the log level of all loggers containing the argument. Hence, "Ass" would match the logger created above. Decision has to be made:

1. One could use the currect logger names and get a more granular log level switching. The different logger names would then need to be documented thoroughly.
2. One could change the getLogger(Class<?> clazz) to the getLogger(String name), and use a common name for all loggers, such as "reviewassistant". It would then be possible to switch all of ReviewAssistant's logger levels with gerrit logging set-level debug reviewassistant.

[nllptr]

I think I prefer method 2. Start simple, and make it more advanced if there is a need. There could even be a combination of the two, for example naming a logger "reviewassistant.reviewers", and another logger "reviewassistant.reviewtime".