Adding target frameworks for net45 & netstandard2.0, removing netstandard1.3

[BrettJaner]

Related to issue #104. Trying to align target frameworks closer to how core Serilog library works. Here is what I think should be done. Let me know your thoughts.

1. Removing targetframework for netstandard1.3 because it's just a superset of netstandard1.1
2. Removing the need to target the full NETStandard.Library when targeting netstandard1.1 (NETStandard.Library ends up a dependency in the nupkg when targeting pre-netstandard2.0)
3. Adding targetframework for net45 to use framework assembly for System.Net.Http when targeting full .NET Framework
4. Adding targetframework for netstandard2.0 to remove need to reference System.Net.Http package