Provide a NuGet package for TensorFlowSharp without precompiled native binaries

[glopesdev]

This would be very useful when using tensorflow for GPU, or when deploying on different *NIX flavors. In these cases, it might be more useful to have separate wrapper and native CPU/GPU native runtime packages, and potentially a meta-package for those who want to install common combinations.

[glopesdev]

This has the further advantage that the nuget package will be trimmed down to around ~700k instead of 150M. When targeting different platforms users will then be able to pick and choose which native libraries they want to include.