Support CoreCLR and EF Core

[trayburn]

Lots of work to do here, but this is the start. This PR will let you track our progress.

Here is some information about how we are proceeding towards Highway.Data v7.0.0:

• Support dnxcore50 and dnx451
• Allow C# 6.0 syntax into the codebase
• Normalize on NUnit as Test Runner of choice, rework all tests to support same
• Move from psake to a new task runner which is cross platform.
  • Currently prototyping gulp
  • Will also prototype cake and possibly fake before a decision is made
• Split Highway.Data.dll into, at a minimum:
  • Highway.Data
  • Highway.Data.InMemory
  • Quite possibly also Highway.Data.Criteria and Highway.Data.Dynamic

That is a good list for now, more to come I'm sure.