Update logging (CocoaLumberjack/NBULog support)

RestKit relies too much in the underlying log framework while some people may want to use a different one (#1393).

This pull request abstracts the actual framework and adds support for NSLog-based logging as an example. Next [CocoaLumberjack](https://github.com/CocoaLumberjack/CocoaLumberjack) and [NBULog](https://github.com/CyberAgent/iOS-NBULog) support will be added.
@carllindberg It would be interesting to see your implementation.

I'm trying to do that here. In other projects I have managed to do it like [this](https://github.com/CyberAgent/iOS-NBUImagePicker/blob/master/Source/NBUImagePickerPrivate.h):

```obj-c
// a) Use NBULog for logging when available
#if __has_include("NBULog.h")

#import "NBULog+NBUImagePicker.h"

#undef  LOG_CONTEXT
#define LOG_CONTEXT NBUIMAGEPICKER_LOG_CONTEXT

#undef  LOG_MODULE
#define LOG_MODULE  NBUIMAGEPICKER_MODULE_DEFAULT

#undef  LOG_LEVEL
#define LOG_LEVEL   [NBULog imagePickerLogLevel]


// b) Else try CocoaLumberjack
#elif __has_include("DDLog.h")

#ifdef DEBUG
    #define NBUImagePickerLogLevel DDLogLevelVerbose
#else
    #define NBUImagePickerLogLevel DDLogLevelWarning
#endif

#define LOG_LEVEL_DEF   NBUImagePickerLogLevel
#import <CocoaLumberjack/DDLog.h>

#define NBULogError(frmt, ...)      DDLogError(frmt, ##__VA_ARGS__)
#define NBULogWarn(frmt, ...)       DDLogWarn(frmt, ##__VA_ARGS__)
#define NBULogInfo(frmt, ...)       DDLogInfo(frmt, ##__VA_ARGS__)
#define NBULogDebug(frmt, ...)      DDLogDebug(frmt, ##__VA_ARGS__)
#define NBULogVerbose(frmt, ...)    DDLogVerbose(frmt, ##__VA_ARGS__)
#define NBULogTrace()               NBULogDebug(@"%@", THIS_METHOD)


// c) Else fallback to NSLog
#else

#ifdef DEBUG
    #define LOG_LEVEL 3
#else
    #define LOG_LEVEL 2
#endif

#define THIS_METHOD                 NSStringFromSelector(_cmd)
#define NBULogError(frmt, ...)      do{ if(LOG_LEVEL >= 1) NSLog((frmt), ##__VA_ARGS__); } while(0)
#define NBULogWarn(frmt, ...)       do{ if(LOG_LEVEL >= 2) NSLog((frmt), ##__VA_ARGS__); } while(0)
#define NBULogInfo(frmt, ...)       do{ if(LOG_LEVEL >= 3) NSLog((frmt), ##__VA_ARGS__); } while(0)
#define NBULogDebug(frmt, ...)      do{ if(LOG_LEVEL >= 4) NSLog((frmt), ##__VA_ARGS__); } while(0)
#define NBULogVerbose(frmt, ...)    do{ if(LOG_LEVEL >= 5) NSLog((frmt), ##__VA_ARGS__); } while(0)
#define NBULogTrace()               NBULogDebug(@"%@", THIS_METHOD)

#endif
```

Simply that detecting what libraries (if any) is available and defining macros for it. No library is required or included.

Moreover it would be great to come up with a [slf4j](http://www.slf4j.org)-like solution that can be used for any project and plugged to any logging solution (https://github.com/CocoaLumberjack/CocoaLumberjack/issues/329).