Change plugin projects to netstandard and sdk style csproj's

[Barsonax]

Changes:

• All pcl projects have been converted to netstandard
• Updated net framework nuspecs that still were using net45 to net472, I think we forgot to do this in the past.
• Moved most of the stuff that was in AssemblyInfo.cs to the csproj itself. With net sdk style projects the default is to put things like version and other metadata in the csproj.
• Updated the nuspecs and also added targetframework groups (this is the same as the default output when you pack a project).

Also solves #743

[Barsonax]

Rebased on latest master, changes are down from 59 to 32 files :)