Prepared to be used as a UPM package

[Cobo3]

I prepared the project so that it can be used as a git UPM package. After the PR, one can simply install this using the following git URL:

https://github.com/cemuka/UnityRuntimeNodeEditor.git?path=/Packages/com.cemuka.runtime-node-editor

Additionally:

• Updated the README.md with instructions on how to install the package
• Update the project to the latest version of Unity (6.4)
• Improved the way the code handles the new input system installation
• Separated the code source code from the examples and added them to the Samples~ folder