2.0.0

Changes discussed in #225. Includes commits from branches [2.0](https://github.com/CocoaLumberjack/CocoaLumberjack/tree/2.0) and [2.0_rebased](https://github.com/CocoaLumberjack/CocoaLumberjack/tree/2.0_rebased).

For now the main changes are to break up `DDLog.h` into `DDLogMacros` and `DDAssertMacros` to ease customizing CocoaLumberjack, and reorganization of files and Demo projects.

For users who just want to use it as it was in 1.x.x just replace `#import <CocoaLumberjack/DDLog.h>` by `#import <CocoaLumberjack/CocoaLumberjack.h>`.