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.
Definitely seems like an alternative to `PeriodicBatchingSink` is needed here - perf with `Task.Factory.StartNew` is going to be nasty, as will the quick consumption of large numbers of `ThreadPool` threads (not to mention the network round-trips :)) .... Wonder what options we have for porting `PBS` to the PCL version?