[Issue 445] Unity Package Manager Distribution

[aaronchapin-tocaboca]

With this change, Unity devs can add this package to their project using the Unity Package Manager. In manifest.json, all that's needed to be added is:

"com.revenantx.litenetlib": "https://github.com/aaronchapin-tocaboca/LiteNetLib.git?path=/LiteNetLib#feature/unity-package-manager",

With the url changing, obviously after this PR has been merged.

A drawback is that the project has to have Unity's .meta files in order to be linked properly, which will likely be annoying for non-Unity devs. So far I haven't found a way around this, but if anyone has a suggestion I'd be willing to try it out!

fixes #445