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.~~

So what is the point or need to undef and define the MarkerName, do you have to set this same MarkerName for each class that it in the same "module". This seems goofy to me, I go like the idea above of `SLInfo("RestKit/Network", "Log message for networking");` That way as a developer of a third party lib I could put all my MarkerName's as new defines in the umbrella header so my app could also use them for cross cutting modules/markers.