First cut on a Serilog integration.

Need to figure out disposal of correlationProperty.
Note: Test project upgraded to .NET 4.5
Updated method summaries
Added Rebus.Serilog.nuspec
Fixed adding properties to Serilog
Updated to Serilog 1.3.1
Switched to MessageContextEstablished
Merge branch 'feature-serilog' of https://github.com/fritsduus/Rebus into feature-serilog
added entry to changelog for serilog logger