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.~~

`markerWithName:@"RestKit"` creates a new marker unless one with that name already exists.

Markers can be nested. For instance `[SLLog markerWithName:@"RestKit/Network/Object Mapping"`:

* Will create marker named `RestKit/Network/Object Mapping`.
* Will nested inside the `[SLLog markerWithName:@"RestKit/Network"]` marker, which will also be nested inside the `RestKit` marker.

The convenience `appMarker` is just a shortcut to the default `[SLLog markerWithName:@"App"]`

The logic to easily retrieve and keep track `allMarkers` is still to be done.