Switch Tracing from log4net to Diagnostics.Trace

[ascheibe]

Hive sandboxing executes code in an AppDomain with restricted privileges. All code that is executed in such an environment is partially trusted. This makes it possible to restrict permissions and define what code is allowed to be executed. log4net contains code that needs fully trusted callers. Because the code in a sandbox is executed partially trusted, log4net can't be used with Hive sandboxing. System.Diagnostics.Trace can be used with Hive sandboxing as it requires no fully trusted caller. Therefore the Logger class in Tracing will be rewritten to use Diagnostics.Trace. Configuration of the Tracing is done in app.config under the <system.diagnostics> Tag. To include this section in HeuristicLab's app.config, ConfigMerger has to be changed to also merge the <system.diagnostics> section. The log4net plugin will be removed from HeuristicLab as it is only used by HeuristicLab.Tracing.