Update unit tests and restructured internals visible to derived projects only.

Switch out unit test framework. Still failing for Appveyor.
Removed latest targeting frameworks for this build. Will revert in next. Satisfies appveyor for PR.
Restructured KeyHandler and internals for ReadLine library. Does not make internals visible externally.
Added visibles to derived projects expecting code signed key.

Tested issue tonerdo#52 and unable to reproduce.
Demo app seems to run fine.