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.~~

For now the libraries macros are the same as the App macros except for the `SLMarkerName` that libraries should set to a different value anyway.

* Sample library code that logs as "Other":
```obj-c

#import "SLLibMacros.h"

// ...

SLInfo(@"Logging as 'Other'");
```

* Sample library code that customizes its marker:
```obj-c

#import "SLLibMacros.h"
#undef  SLMarkerName
#define SLMarkerName @"RestKit/Network"

// ...

SLInfo(@"Logging as 'RestKit/Network'");
```

Apps can do the same with markers such as `App/UI`.