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)

Not sure the merge from `serilog:master` was the cleanest and it needs a review.  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.