Refactored the logger to use PSR-3 and Monolog

Work in progress

TODO:

- [ ] update the [PiwikDebugger plugin](https://github.com/piwik/plugin-PiwikDebugger)
- [ ] update the [SiteMigration plugin](https://github.com/piwik/plugin-SiteMigration)
- [ ] remove more Piwik code to use Monolog classes (for now I've kept most Piwik code to make sure to keep BC on the behavior, now that it's clearer I can try remove some code)
- [ ] simplify the DI config (very complex for now because of BC with INI config)
- [ ] clean up the global.ini config somehow
- [ ] write unit tests (waited before writing them because most code I wrote for the refactoring will be replaced by Monolog)
- [ ] move the log config & classes to a plugin? Now that Piwik Core can use `Psr\Log\LoggerInterface`, the Core (and other plugins) shouldn't care about the implementation/config
- [ ] document using `Psr\Log\LoggerInterface` instead of `Piwik\Log`
- [ ] document using PSR-3 placeholders instead of sprintf
- [ ] document how plugins (or users) can override the default config (e.g. have a plugin to use Logstash for example…)

For the record I do not believe this will be ready for 2.10, I should probably.