Clean up F# OSS build targets files.

[KevinRansom]

1. Use public sign.
2. fix some slightly irritating indentation.
3. Move release and debug speciffic settings to settings.targets
4. Remove TargetFramework support for platforms that are not part of the OSS build.

[dsyme]

For those who want to know what public signing is: #1376 (comment)

[KevinRansom]

Public signing is kind of a loader trick. Delay the sign the dll, and mark the signed bit in the CLR header to signed and the CLR loads the dll fine, except where it would normally verify the assembly, So public signed assemblies can not be used in Partial App Domains or from the GAC.

Official Microsoft Binaries will still be fully signed using the .net framework keypair. This is just a way of removing the administrator and skip verification requirement when building our OSS binaries

Kevin

[OmarTawfik]

This will affect all projects?

[KevinRansom]

VS2014 and Dev15 from Update 2 onwaqrds can both debug portable PDB's, and portable PDBs are much ... much smaller.