First prototype [Work In Progress]

A first try to implement #1:

* A minimal `SLLog` object that keeps track of a `loggerClass` and some common _marker_ logic.
* A `SLLogger` protocol for log framework adapters.
* A `SLMarker` protocol and a `SLSimpleMarker` implementation of that protocol to support context-aware log frameworks.
* Simple `SLMacros` and `SLLibMacros` to be used by Apps and 3rd party libraries respectively.

The façade is "one way" only:

* Wrap log calls to direct them to log frameworks
* ~~Wrap log frameworks configuration/usage.~~

Default levels.

Because this will probably not be extensible in the future we could add some more and then let log frameworks "merge" log levels when not available.