New DDAssert macros

I started to use `NSAssert` and realized the need for some CocoaLumberjack customization as first asked in #23 by @hartbit.

`DDAssert` macros besides calling `NSAssert` log with `DDLogError` so we get output even when  assertions are disabled (default Xcode settings for Release builds). Also these macros have a more flexible format specifier than Apple's ones.

The only thing that annoys me is that somehow `NSAssert` silences some Analyzer warnings that `DDAssert` doesn't.