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.
Might be good to copy the cut-down AssemblyInfo.cs style from the other sinks.