migueldeicaza
Add support for .Net 4.5.
This PR adds support for .Net 4.5 to TensorFlowSharp. Along the way I converted all of the build projects to the new, much more concise project format which also allows the projects to target multiple frameworks. I also changed the main project to copy the native tensorflow binaries for the current platform to its output directory to make it easier to build and test and added some documentation about how to build and run the tests to the new file BUILD.md.
Unfortunately with these changes 6 of the csharp tests fail, but the same tests fail on all 3 platforms (.net 4.5, .net 4.6 and newer and netstandard 2.0), and I was unable to get all tests to pass before the changes, so I don't think there is a regression.
