Add required bridged cast

NSLogger-CocoaLumberjack-connector/DDNSLoggerLogger.m:63:50: error: cast of
Objective-C pointer type 'NSString *' to C pointer type 'CFStringRef' (aka
'const struct __CFString *') requires a bridged cast

Issue #9
Merge pull request #10 from kolyuchiy/fix-bridged-cast

Add required bridged cast