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.
Using `Serilog` as the root namespace makes it easier to put the extension methods into it (reducing need for extra `using` stmts for users)