Refactored stream related stuff

- simpler, polymorphic hierarchy-based, approach
- less bitty conditionals spread across the code
- all resolved up-front so now config class is immutable
(it had evolved the way it was and in need of a clean-up sweep for a long time)

include/internal/catch_context_impl.hpp

@@ -95,14 +95,6 @@ namespace Catch {
         return getCurrentMutableContext();
     }
 
-    Stream createStream( std::string const& streamName ) {
-        if( streamName == "stdout" ) return Stream( Catch::cout().rdbuf(), false );
-        if( streamName == "stderr" ) return Stream( Catch::cerr().rdbuf(), false );
-        if( streamName == "debug" ) return Stream( new StreamBufImpl<OutputDebugWriter>, true );
-
-        throw std::domain_error( "Unknown stream: " + streamName );
-    }
-
     void cleanUpContext() {
         delete currentContext;
         currentContext = CATCH_NULL;