Add dotnet core rc2 support and split into multiple versions/flavors

[StefH]

This pull request is to convert this project into the new dotnet rc2 project structure which adds support for more frameworks like NetStandard 1.3

Also in relation to this question : #28 , I did create 3 projects:

1. LinqKit : the original code but this will compile from NET 3.5 until NetStandard 1.3, and has no dependency on EntityFramework.
2. LinqKit.EntityFramework : this also based on your original code, but this one supports NET 4.5 until 4.6.1 and includes a dependency to EntityFramework 6.1.3
3. LinqKit.Microsoft.EntityFrameworkCore : this also based on your original code, but this one supports NET 4.5.1 until NetStandard 1.3 and includes a dependency to Microsoft.EntityFrameworkCore 1.0.0-rc2-final

So can you please accept this PR so that your very handy project can be used in multiple ways, and can support more frameworks and versions from EF?

Note that for last two packages I've also created a NuGet, however, if you accept this PR, and upload a new LinqKit based on my first project (with correct version), I can transfer ownership from these twp packages to you.

(I also saw that @dopare copied this whole project and started a new at https://github.com/dopare/LinqKit.Core which is also available on NuGet, so maybe we can also merge something in that erea)

[Thorium]

Thanks!

[Thorium]

The old build fails after merge: https://ci.appveyor.com/project/Thorium/linqkit/build/1.0.15