Convert to SDK style project and update dependencies

[ericstj]

I commented in https://github.com/devtrends/Unity.WebAPI/pull/21 about how just updating newtonsoft wasn't the correct thing, so I thought I'd give a try at updating this project myself.

I converted to SDK style and retained the same package structure (probably it could be modernized to use ContentFiles, but I'm not super familiar with that).

I also made it do public signing - which is "fake signing" with just an exported public key. If you were to accept this change and you have the real keypair, you'd want to make sure you build and override the two properties /p:AssemblyOriginatorKeyFile=<pathToRealSNK> /p:PublicSign=false