"Fix" conflicts with 3rd party libraries using CocoaLumberjack

As discussed in #162, there's no solution to sharing `ddLogLevel` between libraries and the client application:

* Apps/libraries may define `ddLogLevel` as a const, extern, int, a function returning an int, etc. Each one incompatible with the other.

* Even more, if apps/libraries used a compatible and shared `ddLogLevel` there would be no way to manage separately log levels from each one.

The solution is to define a `LOG_LEVEL_DEF` by default equal to `ddLogLevel` so apps can continue to work as usual.

3rd party libraries simple redefine `LOG_LEVEL_DEF` to something different:

```obj-c
// #undef LOG_LEVEL_DEF // Undefine first only if needed
#define LOG_LEVEL_DEF myLibLogLevel
```

3rd party frameworks **need** to be updated.