[WIP] add .NET Core support

[enricosada]

ref #199

• add .NET Core netstandard1.5 support using .NET CLI
• add build in appveyor
• add build in travis
• use dotnet mergenupkg to add netstandard1.5 to current FsCheck nupkg
• run command inside build.fsx using fake targets

To create the package:

dotnet restore
cd src\FsCheck
dotnet pack --configuration Release

this pr it's used in tutorial https://github.com/enricosada/fsharp-dotnet-cli-samples/wiki/Migrate-fsproj-To-.NET-CLI

The added test project is not needed, but is nice to check .net core.
It can be removed when xunit tests works

[kurtschelfthout]

I'm just wondering with all the recent churn around .NET core if this PR is still the way forward? (I plan to take some time to look into this in more detail shortly, just wondering if this is still a reasonable place to look). Thanks.

[enricosada]

yes, it is. I'll update it for .net core sdk preview2 and .net core rtm ( netstandard1.6 ).
maybe for the tomorrow c4fsharp webcast, or after that.
now the remaining points are a routine job, already done in other projects, so the worflow is minimal (changes and maintance) and stable.
That mean, after the usual build BuildPackage or build Nuget the package has the .net core inside (if .net core sdk is installed, otherwise .net core stuff skipped)

[kurtschelfthout]

Great, just checking. Don't let me rush you!

[enricosada]

closing in favor of #293