Allow building strong named (signed) assemblies as weak named ones

Hello,

the assemblies can now be build with strong names (signed) as well.
Maybe it is better to add the signed versions to the nuget packeage - but that's on your decision.

Also feel free to completely change/revert/criticise any of the changes I have done. 

Remark: The changes all have been done using the git editor (had no visual studio at hand) but it should work. If not, please tell me.

Changes:
- add: added missing guid attributes to assemblyinfo.cs files
- add: added a key for signing assemblies to the solution folder
- add: a Debug_signed and Release_signed configurations have been added to the solution and the projects

Regards,
Thomas