[WIP] .Net Standard 2.0 support

[GerardSmit]

Thanks for this repo, it helps kick-starting my project quite a bit. The only issue I have is that SharpChakra not targeting .Net Standard 2.0 (for .Net Core).

This PR adds support for .Net Standard 2.0:

• Added Native64Linux for Linux support (not tested yet).
• Added Native64Mac for Mac support (not tested yet).
• Added native runtimes to the repo:
  • For .Net Framework only the Windows (x86, x64) natives are copied to the output folder.
  • For .Net Standard all the natives are copied to the output folder.
• Removed NuGet reference "Microsoft.ChakraCore" since the natives are include in the repo.

Additionally I changed the structure a little bit:

• Moved src\Sample* to samples.
• Added tests for testing (need create more tests and need to add Travis/AppVeyor).

If you don't want the stucture to be changed feel free to close this PR and get the code for .Net Standard from my fork.

[GerardSmit]

I've found some issues I need to fix. I'll push them this week.