Added SourceContext to the Serilog Logger

Without this there is no way to capture the name / context (commonly the type name) provided by a facility while wiring a logger.

AFAIK the .ForContext("SourceContext", name); Has the same behavior as .ForContext<T>() in Serilog. Which is how they capture context and which translates into things like Loggername when you use other sinks in Serilog. I've tested this with a Serilog-NLogSink being injected via the castle facility.

If someone has a better idea for the "SourceContext" constant? Maybe a constructor parameter? It's a cross project dependency :(