Add .Net Core support

[AlexGhiondea]

This PR adds support for building the library to target .NET Standard 1.3.

In order to maintain compat with older version of .NET Framework, the library is cross-compiled to .NET Framework 4.5 as well.

Changes included:

• Create a new project that targets .NET Standard 1.3. Add file-links to the source code files from the Portable project.
  • This maintains file history (although git is rather good at tracking files after moves) and keeps a single copy of the source files (important).
  • The product will also produce a NuGet package (v3.0) with the .NET Framework 4.5 and .NET Standard 1.3 assets.
  • This also keeps the Portable library building for now.
• Create a new unit test project. The unit test project targets .NET Framework 4.6 and consumes the .NET Standard 1.3 library (to test the new version of the code).
  • Once the test dependencies are updated to support .NET Core we should create a .NET Core unit test project for this.
  • The test files are also file-linked in to keep the portable tests working for now.
• There are a couple of places where the code needed to be slightly changed to accommodate for the .NET Standard 1.3 available APIs. Those places are #if/def-ed under NETSTANDARD1_3

This v3.0 branch should merge this PR once it is approved: #223

[WestDiscGolf]

This is awesome :)

[AlexGhiondea]

Thanks! :)

[JeffDalby]

Very cool. Looking forward to testing it out.

[amammay]

Going to give this a test through! Awesome!

[joshfitabase]

Thanks for your work on this @AlexGhiondea !