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.~~

Macros that an App would **optionally** use if they plan to also rely on SLF4C.