Support json output fields defined in NLog configuration

Support json fields defined as Layout in NLog config.

<target xsi:type="RabbitMQ" useJSON="true" ...>
<field name="threadid" layout="${threadid}"/>
<field name="instance" layout="Node-12"/>
</target>

Configuration defined fields are overridden if same field occurr as
LogEvent property.
Update NLog.config
Removed configuration x86. Everything now compiles as AnyCPU
Merge pull request #11 from ErikRydgren/master

Support json fields defined as Layout in NLog config.