* LogLevel.cs: Added log level

* LogLevelConverterExtension.cs: added converter

* ILogger.cs:
* NLogAdapter.cs:
* ILoggerManager.cs:
* NLogManagerAdapter.cs: added methods and properties

* DocToMarkdown.Common.csproj: added classes
  LogLEvelConverterExtension and LogLevel
* NLogAdapterTest.cs:
* NLogManagerAdapterTest.cs:
* LogLevelConverterExtensionTest.cs: added test

* DocToMarkdown.Common.Tests.csproj: added nuget package: NLog
Added test LogLevelConverterExtensinTest

* packages.config: added nuget package NLog
added shutdown for loggerManager