[pull] master from exodrifter:master - #1

Added missing Microsoft.Scripting.Metadata.dll and removed
IronPython.Wpf.dll.

Fixes #12

All assemblies in the `UnityEngine` and `UnityEditor` namespaces are now
imported automatically when using `UnityPython.CreateEngine`.

This fixes problems where not all types were available to import inside
of a Python script, such as Rigidbody.

The requirement to change scripting runtime version to .NET 4.X is not
neccessary since the DLLs provided work in .NET 2.X.

Add platform requirements for this library and explicitly state which
Python version it supports, which are both frequent questions in issues
opened for this respository.

As discovered in #34, the API compatability level still needs to be
changed to .NET 4.X in order to work when the application is built.

This reverts commit 79599ab.

Also added a third party notices file as suggested in the official Unity
package documentation (https://docs.unity3d.com/Manual/cus-legal.html)
and updated the README to add the package manager installation
instructions.