add support for NLog 3(.1)

In my test project this fixes #51.
Notice: .Net 2.0 is no longer supported by NLog 3(.1).

(I locally referenced the created NuGet-packages and added the following to my app.config:

<configSections>
    <sectionGroup name="common">
      <section name="logging" type="Common.Logging.ConfigurationSectionHandler, Common.Logging"/>
    </sectionGroup>
    <section name="nlog" type="NLog.Config.ConfigSectionHandler, NLog" />
  </configSections>

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="NLog" publicKeyToken="5120e14c03d0593c" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.1.0.0" newVersion="3.1.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>

<common>
    <logging>
      <factoryAdapter type="Common.Logging.NLog.NLogLoggerFactoryAdapter, Common.Logging.NLog31">
        <arg key="configType" value="INLINE" />
      </factoryAdapter>
    </logging>
  </common>

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" autoReload="true">

    <targets async="true">

      <target name="TraceLogger" xsi:type="File" layout="${pad:padding=-24:inner=${longdate}}||${pad:padding=-5:inner=${level:uppercase=true}}||${logger}||${message:whenEmpty=(no message)}${onexception:${newline}${exception:format=ToString,StackTrace:maxInnerExceptionLevel=10}}" fileName="TraceLogfile.txt" archiveFileName="archive/TraceLogfile.{####}.txt" archiveEvery="Day" archiveNumbering="Sequence" maxArchiveFiles="7" footer="----------|${pad:padding=-24:inner=${longdate}}|Ende Programm" concurrentWrites="false" keepFileOpen="true" autoFlush="true" encoding="utf-8" createDirs="true" lineEnding="CRLF" />

    </targets>
    <rules>
      <logger name="*" minlevel="Trace" writeTo="TraceLogger" />
    </rules>
  </nlog>
)