Enable development on *nix environments

[jburger]

This PR is intended to provide appropriate framework targeting and compiler directives to allow developers on non-windows environments to work against the majority of the codebase.

[johnsimons]

@jburger given net45 is only used in samples, I think we can safely upgrade those, or maybe even just have samples in netcoreapp?

[johnsimons]

Given how small this project is, do we even need cake at all?

[johnsimons]

The CertificateGenerator seems to be just for convenience, it doesn't seem to be used in cake either. I think we can just remove it? maybe replace with multi-plat instructions in readme?

[jburger]

Great points there @johnsimons

maybe even just have samples in netcoreapp

I like this idea, I'll give it a go. If others need a working net4.5 example perhaps that can be provided via the readme at a later time.

Given how small this project is, do we even need cake at all?

I suspect not! I'll see what its doing and reason about minimizing that dependency

The CertificateGenerator seems to be just for convenience

Yeah I personally prefer using openssl though I haven't tried in the context of Halibut, I'll give that a try and see what comes of it.

[jburger]

• ditched 4.5 in favour of 4.7.2 where necessary
• samples are on netcoreapp2.2
• decided to fix the cake bootstrap instead of rewrite the build scripts altogether.
  • I think there is value in keeping our build tools as consistent as possible across our repositories
  • Would have bumped into similar issues anyway - e.g. JetBrains profiling tools on dotnet core
• I've left the cert generator there in case people aren't willing or able to use openssl or powershell
  • provided examples of how to generate self signed certificates using openssl in readme
  • provided script to make that a one liner on os x / linux