Changed ResolveLogProvider to check for Serilog

When using SeriLog with Log4Net or any other logging provider, LibLog
would use those providers first over using Serilog.
Updated pp and added a test

Added a test to make sure that Serilog is returned over Log4Net,.
Merge pull request #10 from ChrisHimsworth/master

Changed ResolveLogProvider to check for Serilog