Implemented Logger.Swallow and Logger.SwallowAsync methods
Merge branch 'master' of github.com:robertvazan/NLogSwallow into NLogSwallow

Conflicts:
	src/NLog/Logger.cs
	tests/NLog.UnitTests/LoggerTests.cs
	tests/NLog.UnitTests/NLogTestBase.cs
Merge branch 'NLogSwallow' into umaster