Add Unit Tests to MemoryTarget class

Conflicts:
	tests/NLog.UnitTests/NLog.UnitTests.mono2.csproj
	tests/NLog.UnitTests/NLog.UnitTests.monodevelop.csproj
	tests/NLog.UnitTests/NLog.UnitTests.netfx35.csproj
	tests/NLog.UnitTests/NLog.UnitTests.netfx40.csproj
	tests/NLog.UnitTests/NLog.UnitTests.netfx45.csproj
	tests/NLog.UnitTests/NLog.UnitTests.sl4.csproj
	tests/NLog.UnitTests/NLog.UnitTests.sl5.csproj
Added Unit Tests for InternalLogger class

The code coverage increased to 88%. The remaining code is in the various
GetSetting() private methods.

Conflicts:
	tests/NLog.UnitTests/Fluent/LogBuilderTests.cs
Unit Tests added for LogManager class

Unit Tests added for LogManager class to cover EnableLogging(),
DisableLogging() and IsLoggingEnabled() methods.