Added support for hidden/blacklisted assemblies

This feature allows the consumer to add specific assemblies to a "blacklist". All added assemblies will be skipped when NLog is trying to find out the calling method. This scenario is sometimes useful, if the consumer don't want to show up a specific assembly in log files. Another scenario would be if the consumer want's to wrap NLog in an own assembly and don't want show up the callsite from the wrapper assembly.

This feature is a small extension for the existing SkipAssembly method inside of the LoggerImpl class to skip unwanted assemblies like mscorlib.