Implement an already existing error/event logger

Requirements:
- Events shall be stored in a circular list (provided by external class)
- Possibility to log towards the serial monitor, SD-card (provided by external class) and LCD screen (To be done)
- For critical event use led blinking schemes (see #35)
- Possibility to choose logging frequency (flush() call should be sampled)
- Logs should have a severity:
- ERROR,
- CRITICAL (see #32),
- WARNING,
- INFO,
- DEBUG (Only activated when compilation argument DEBUG is set)

The overall design of this class structure is explained [here](https://github.com/margrevm/trollbot/wiki/Software-design).

Formatting:
- On-board time is logged with every message
- Each type of log should prefixed with a marker. Example for debug log : *[DEBUG]*

Steps:
- [ ] Implement the class
- [ ] Write integration tests
- [ ] Update documentation