Take 2 - New PCL & FullNetFx assembly for Splunk to align with 2.0 Beta PCL Splunk package

Two new assemblies, once PCL targeting HTTP, one Net 4.5 to add TCP & UDP.

`Serliog.Sinks.Splunk` (PCL)
`Serliog.Sinks.Splunk.FullNetFx` (4.5)

UDP may need to change to periodic sink. Also, how is best to add wiki updates once PR is finalised?

Once PR ok, happy to update samples accordingly.
Will add, however "under the hood" Splunk will expect this objects to connect.  Wanted to respect the SDK but happy for a *simple* overload